[clang][cmake] Force CMAKE_LINKER for multistage build in case of BOOTSTRAP_LLVM_ENABLE_LLD and MSVC

The issue with LLVM_ENABLE_LLD is that it just passes -fuse-ld=lld
to compiler/linker options which makes sense only for those platforms
where cmake invokes a compiler driver for linking. On Windows (MSVC) cmake
invokes the linker directly and requires CMAKE_LINKER to be specified
otherwise it defaults CMAKE_LINKER to be link.exe.

This patch allows BOOTSTRAP_LLVM_ENABLE_LLD to set CMAKE_LINKER in two cases:
* if building for host Windows,
* if crosscompiling for target Windows.

It also skips adding '-fuse-ld=lld' to make lld-link not warning
about 'unknown argument'.

This fixes build with `clang/cmake/caches/DistributionExample.cmake`
on Windows.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D80873

clang/CMakeLists.txt

@@ -753,6 +753,14 @@ if (CLANG_ENABLE_BOOTSTRAP)
     -DCMAKE_ASM_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/${C_COMPILER}
     -DCMAKE_ASM_COMPILER_ID=Clang)
 
+  # cmake requires CMAKE_LINKER to be specified if the compiler is MSVC-like,
+  # otherwise it defaults the linker to be link.exe.
+  if(BOOTSTRAP_LLVM_ENABLE_LLD)
+    if((WIN32 AND NOT BOOTSTRAP_CMAKE_SYSTEM_NAME) OR BOOTSTRAP_CMAKE_SYSTEM_NAME STREQUAL "Windows")
+      set(${CLANG_STAGE}_LINKER -DCMAKE_LINKER=${LLVM_RUNTIME_OUTPUT_INTDIR}/lld-link${CMAKE_EXECUTABLE_SUFFIX})
+    endif()
+  endif()
+
   if(BOOTSTRAP_CMAKE_SYSTEM_NAME)
     set(${CLANG_STAGE}_CONFIG -DLLVM_CONFIG_PATH=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-config)
     set(${CLANG_STAGE}_TABLEGEN

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -261,7 +261,12 @@ if( LLVM_ENABLE_LLD )
   if ( LLVM_USE_LINKER )
     message(FATAL_ERROR "LLVM_ENABLE_LLD and LLVM_USE_LINKER can't be set at the same time")
   endif()
+  # In case of MSVC cmake always invokes the linker directly, so the linker
+  # should be specified by CMAKE_LINKER cmake variable instead of by -fuse-ld
+  # compiler option.
+  if ( NOT MSVC )
     set(LLVM_USE_LINKER "lld")
+  endif()
 endif()
 
 if( LLVM_USE_LINKER )