[CMake][Compiler-rt] Make it possible to configure standalone compiler-rt without `LLVMConfig.cmake`.

Previously it wasn't possible to configure a standalone compiler-rt
build if the `LLVMConfig.cmake` file isn't present in a shipped
toolchain.

This patch adds a fallback behaviour for when `LLVMConfig.cmake` is not
available in the toolchain being used for configure. The fallback
behaviour mocks out the bare minimum required to make a configure
succeed when the host is Darwin. Support for other platforms could
be added in future patches.

The new code path is taken either in one of the following cases:

* `llvm-config` is not available.
* `llvm-config` is available but it provides an invalid path for the CMake files.

The motivation here is to be able to generate the compiler-rt lit test
suites for an arbitrary LLVM toolchain and then run the tests against
it.

The invocation to do this looks something like.

```
CC=/path/to/cc \
CXX=/path/to/c++ \
cmake \
  -G Ninja \
  -DLLVM_CONFIG_PATH=/path/to/llvm-config \
  -DCOMPILER_RT_INCLUDE_TESTS=ON \
  /path/to/llvm-project/compiler-rt

 # Note we don't compile compiler-rt in this workflow.
bin/llvm-lit -v test/path/to/generated/test_suite
```

A possible alternative approach is to configure the
`cmake/modules/LLVMConfig.cmake.in` file in the LLVM source tree
and then include it. This approach was not taken because it is more
complicated.

An interesting side benefit of this patch is that it is now
possible to configure on Darwin without `llvm-config` being available
by configuring with `-DLLVM_CONFIG_PATH=""`. This moves us a step
closer to a world where no LLVM build artefacts are required to
build compiler-rt.

rdar://76016632

Differential Revision: https://reviews.llvm.org/D99621

compiler-rt/cmake/Modules/CompilerRTMockLLVMCMakeConfig.cmake

@@ -0,0 +1,75 @@
+# This macro mocks enough of the changes `LLVMConfig.cmake` makes so that
+# compiler-rt can successfully configure itself when a LLVM toolchain is
+# available but the corresponding CMake build files are not.
+#
+# The motivation for this is to be able to generate the compiler-rt
+# lit tests suites and run them against an arbitrary LLVM toolchain
+# which doesn't ship the LLVM CMake build files.
+macro(compiler_rt_mock_llvm_cmake_config)
+  message(STATUS "Attempting to mock the changes made by LLVMConfig.cmake")
+  compiler_rt_mock_llvm_cmake_config_set_cmake_path()
+  compiler_rt_mock_llvm_cmake_config_set_target_triple()
+  compiler_rt_mock_llvm_cmake_config_include_cmake_files()
+endmacro()
+
+macro(compiler_rt_mock_llvm_cmake_config_set_cmake_path)
+  # Point `LLVM_CMAKE_PATH` at the source tree in the monorepo.
+  set(LLVM_CMAKE_PATH "${LLVM_MAIN_SRC_DIR}/cmake/modules")
+  if (NOT EXISTS "${LLVM_CMAKE_PATH}")
+    message(FATAL_ERROR "LLVM_CMAKE_PATH (${LLVM_CMAKE_PATH}) does not exist")
+  endif()
+  list(APPEND CMAKE_MODULE_PATH "${LLVM_CMAKE_PATH}")
+  message(STATUS "LLVM_CMAKE_PATH: \"${LLVM_CMAKE_PATH}\"")
+endmacro()
+
+function(compiler_rt_mock_llvm_cmake_config_set_target_triple)
+  # Various bits of compiler-rt depend on the `TARGET_TRIPLE`variable being
+  # defined. This function tries to set a sensible value for the variable.
+  # This is a function rather than a macro to avoid polluting the variable
+  # namespace.
+  set(COMPILER_OUTPUT "")
+
+  # If the user provides `COMPILER_RT_DEFAULT_TARGET_ONLY` and `CMAKE_C_COMPILER_TARGET`
+  # (see `construct_compiler_rt_default_triple`) then prefer that to examining the
+  # compiler.
+  if (COMPILER_RT_DEFAULT_TARGET_ONLY)
+    if (NOT "${CMAKE_C_COMPILER_TARGET}" STREQUAL "")
+      message(STATUS
+        "Using CMAKE_C_COMPILER_TARGET (${CMAKE_C_COMPILER_TARGET}) as TARGET_TRIPLE")
+    endif()
+    set(COMPILER_OUTPUT "${CMAKE_C_COMPILER_TARGET}")
+  endif()
+
+  # Try asking the compiler for its default target triple.
+  set(HAD_ERROR FALSE)
+  if ("${COMPILER_OUTPUT}" STREQUAL "")
+    if ("${CMAKE_C_COMPILER_ID}" MATCHES "Clang|GNU")
+      # Note: Clang also supports `-print-target-triple` but gcc doesn't
+      # support this flag.
+      execute_process(
+        COMMAND "${CMAKE_C_COMPILER}" -dumpmachine
+        RESULT_VARIABLE HAD_ERROR
+        OUTPUT_VARIABLE COMPILER_OUTPUT
+        OUTPUT_STRIP_TRAILING_WHITESPACE)
+    else()
+      message(FATAL_ERROR
+        "Fetching target triple from compiler \"${CMAKE_C_COMPILER_ID}\" "
+        "is not implemented.")
+    endif()
+  endif()
+
+  if (HAD_ERROR)
+    message(FATAL_ERROR "Fetching target triple from compiler failed")
+  endif()
+  set(TARGET_TRIPLE "${COMPILER_OUTPUT}")
+  message(STATUS "TARGET_TRIPLE: \"${TARGET_TRIPLE}\"")
+  if ("${TARGET_TRIPLE}" STREQUAL "")
+    message(FATAL_ERROR "TARGET_TRIPLE cannot be empty")
+  endif()
+  set(TARGET_TRIPLE "${TARGET_TRIPLE}" PARENT_SCOPE)
+endfunction()
+
+macro(compiler_rt_mock_llvm_cmake_config_include_cmake_files)
+  # Some compiler-rt CMake code needs to call code in this file.
+  include("${LLVM_CMAKE_PATH}/AddLLVM.cmake")
+endmacro()

compiler-rt/cmake/Modules/CompilerRTUtils.cmake

@@ -283,6 +283,7 @@ macro(load_llvm_config)
       "You are not using the monorepo layout. This configuration is DEPRECATED.")
   endif()
 
+  set(FOUND_LLVM_CMAKE_PATH FALSE)
   if (LLVM_CONFIG_PATH)
     execute_process(
       COMMAND ${LLVM_CONFIG_PATH} "--obj-root" "--bindir" "--libdir" "--src-root" "--includedir"
@@ -372,9 +373,15 @@ macro(load_llvm_config)
       set(LLVM_CMAKE_PATH "${LLVM_BINARY_DIR_CMAKE_STYLE}/lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm")
     endif()
 
-    list(APPEND CMAKE_MODULE_PATH "${LLVM_CMAKE_PATH}")
-    # Get some LLVM variables from LLVMConfig.
-    include("${LLVM_CMAKE_PATH}/LLVMConfig.cmake")
+    if (EXISTS "${LLVM_CMAKE_PATH}")
+      list(APPEND CMAKE_MODULE_PATH "${LLVM_CMAKE_PATH}")
+      # Get some LLVM variables from LLVMConfig.
+      include("${LLVM_CMAKE_PATH}/LLVMConfig.cmake")
+      set(FOUND_LLVM_CMAKE_PATH TRUE)
+    else()
+      set(FOUND_LLVM_CMAKE_PATH FALSE)
+      message(WARNING "LLVM CMake path (${LLVM_CMAKE_PATH}) reported by llvm-config does not exist")
+    endif()
 
     set(LLVM_LIBRARY_OUTPUT_INTDIR
       ${LLVM_BINARY_DIR}/${CMAKE_CFG_INTDIR}/lib${LLVM_LIBDIR_SUFFIX})
@@ -393,6 +400,15 @@ macro(load_llvm_config)
                     "the `llvm-project` repo. "
                     "This will be treated as error in the future.")
   endif()
+
+  if (NOT FOUND_LLVM_CMAKE_PATH)
+    # This configuration tries to configure without the prescence of `LLVMConfig.cmake`. It is
+    # intended for testing purposes (generating the lit test suites) and will likely not support
+    # a build of the runtimes in compiler-rt.
+    include(CompilerRTMockLLVMCMakeConfig)
+    compiler_rt_mock_llvm_cmake_config()
+  endif()
+
 endmacro()
 
 macro(construct_compiler_rt_default_triple)