[llvm][cmake] Make `install_symlink` workflow work with absolute install dirs

If `CMAKE_INSTALL_BINDIR` is a different absolute path per project, as
it is with NixOS when we install every package to its own prefix, the
old way fails when the absolute path gets prepended with `CMAKE_INSTALL_PREFIX`.

Using `extend_path` from the install-time script isn't really possible, so we just make the caller responsible for making the path absolute instead.

Also fix one stray `bin` -> `CMAKE_INSTALL_BINDIR`

Reviewed By: sebastian-ne

Differential Revision: https://reviews.llvm.org/D101070

llvm/cmake/modules/AddLLVM.cmake

@@ -1,4 +1,5 @@
 include(GNUInstallDirs)
+include(ExtendPath)
 include(LLVMDistributionSupport)
 include(LLVMProcessSources)
 include(LLVM-Config)
@@ -1953,7 +1954,7 @@ endfunction()
 function(add_lit_testsuites project directory)
   if (NOT LLVM_ENABLE_IDE)
     cmake_parse_arguments(ARG "EXCLUDE_FROM_CHECK_ALL" "FOLDER" "PARAMS;DEPENDS;ARGS" ${ARGN})
-    
+
     if (NOT ARG_FOLDER)
       set(ARG_FOLDER "Test Subdirectories")
     endif()
@@ -2009,11 +2010,14 @@ function(llvm_install_library_symlink name dest type)
 
   set(output_dir lib${LLVM_LIBDIR_SUFFIX})
   if(WIN32 AND "${type}" STREQUAL "SHARED")
-    set(output_dir bin)
+    set(output_dir "${CMAKE_INSTALL_BINDIR}")
   endif()
 
+  # `install_symlink` needs an absoute path.
+  extend_path(output_dir "${CMAKE_INSTALL_PREFIX}" "${output_dir}")
+
   install(SCRIPT ${INSTALL_SYMLINK}
-          CODE "install_symlink(${full_name} ${full_dest} ${output_dir})"
+          CODE "install_symlink(\"${full_name}\" \"${full_dest}\" \"${output_dir}\")"
           COMPONENT ${component})
 
 endfunction()
@@ -2048,8 +2052,11 @@ function(llvm_install_symlink project name dest)
     set(full_dest llvm${CMAKE_EXECUTABLE_SUFFIX})
   endif()
 
+  # `install_symlink` needs an absoute path.
+  extend_path(output_dir "${CMAKE_INSTALL_PREFIX}" "${${project}_TOOLS_INSTALL_DIR}")
+
   install(SCRIPT ${INSTALL_SYMLINK}
-          CODE "install_symlink(${full_name} ${full_dest} ${${project}_TOOLS_INSTALL_DIR})"
+          CODE "install_symlink(\"${full_name}\" \"${full_dest}\" \"${output_dir}\")"
           COMPONENT ${component})
 
   if (NOT LLVM_ENABLE_IDE AND NOT ARG_ALWAYS_GENERATE)

llvm/cmake/modules/LLVMInstallSymlink.cmake

@@ -1,22 +1,26 @@
 # We need to execute this script at installation time because the
 # DESTDIR environment variable may be unset at configuration time.
 # See PR8397.
+#
+# `outdir` must be an absolute path. This module gets a very reduced
+# `CMAKE_MODULE_PATH` so it is easier to make the caller the responsible
+# for this.
 
 include(GNUInstallDirs)
 
 function(install_symlink name target outdir)
   set(DESTDIR $ENV{DESTDIR})
-  set(bindir "${DESTDIR}${CMAKE_INSTALL_PREFIX}/${outdir}")
+  set(outdir "${DESTDIR}${outdir}")
 
   message(STATUS "Creating ${name}")
 
   execute_process(
     COMMAND "${CMAKE_COMMAND}" -E create_symlink "${target}" "${name}"
-    WORKING_DIRECTORY "${bindir}" ERROR_VARIABLE has_err)
+    WORKING_DIRECTORY "${outdir}" ERROR_VARIABLE has_err)
   if(CMAKE_HOST_WIN32 AND has_err)
     execute_process(
       COMMAND "${CMAKE_COMMAND}" -E copy "${target}" "${name}"
-      WORKING_DIRECTORY "${bindir}")
+      WORKING_DIRECTORY "${outdir}")
   endif()
 
 endfunction()