[CMake] Use compiler-rt location instead of resource directory to find clang-cls runtime directory

[zero9178]

Summary

The current cmake script attempts to add the path containing clangs various runtime systems by getting the resource directory and then appending the hardcoded value /lib/windows to it. This works for a normal clang-cl build but fails for a build of clang using LLVM_ENABLE_PER_TARGET_RUNTIME_DIR, such as the builds from llvm/runtimes.

This patch instead uses -print-libgcc-file-name in conjunction with --rtlib=compiler-rt, and instead adds the containing directory as library path.

For non per-target runtime directory builds, such as the release builds, there is no change. Even if the builtins library were to be deleted or moved it would output the same path as before.
For per-target runtime builds that also have the builtins library, this now finds the correct directory containing all of clang runtime libraries.

Only case still not handled by this change, is if a per-target runtime directory build is used, but the builtins library was not built.
I believe that is the best we can do for now however, without modifying clang.

Example command line outputs:
LLVM 11 binaries distributed by LLVM:

C:\Program Files\LLVM\bin>clang-cl /clang:-print-libgcc-file-name /clang:--rtlib=compiler-rt
C:\Program Files\LLVM\lib\clang\11.0.1\lib\windows\clang_rt.builtins-x86_64.lib

Per-target runtime dir build of clang:

clang-cl /clang:-print-libgcc-file-name /clang:--rtlib=compiler-rt
C:\clang\Default\lib\clang\11.1.0\lib\x86_64-pc-windows-msvc\clang_rt.builtins.lib

[phosek]

LGTM

llvm/cmake/modules/HandleLLVMOptions.cmake

if (LLVM_BUILD_INSTRUMENTED)
   endif()
 endif()
 
 # When using clang-cl with an instrumentation-based tool, add clang's library
 # resource directory to the library search path. Because cmake invokes the
 # linker directly, it isn't sufficient to pass -fsanitize=* to the linker.
 if (CLANG_CL AND (LLVM_BUILD_INSTRUMENTED OR LLVM_USE_SANITIZER))
   execute_process(
-    COMMAND ${CMAKE_CXX_COMPILER} /clang:-print-resource-dir
-    OUTPUT_VARIABLE clang_resource_dir
+    COMMAND ${CMAKE_CXX_COMPILER} /clang:-print-libgcc-file-name /clang:--rtlib=compiler-rt
+    OUTPUT_VARIABLE clang_compiler_rt_file
     ERROR_VARIABLE clang_cl_stderr
     OUTPUT_STRIP_TRAILING_WHITESPACE
     ERROR_STRIP_TRAILING_WHITESPACE
     RESULT_VARIABLE clang_cl_exit_code)
   if (NOT "${clang_cl_exit_code}" STREQUAL "0")
     message(FATAL_ERROR
       "Unable to invoke clang-cl to find resource dir: ${clang_cl_stderr}")
   endif()
-  file(TO_CMAKE_PATH "${clang_resource_dir}" clang_resource_dir)
-  append("/libpath:${clang_resource_dir}/lib/windows"
+  file(TO_CMAKE_PATH "${clang_compiler_rt_file}" clang_compiler_rt_file)
+  get_filename_component(clang_runtime_dir "${clang_compiler_rt_file}" DIRECTORY)
+  append("/libpath:${clang_runtime_dir}"
     CMAKE_EXE_LINKER_FLAGS
     CMAKE_MODULE_LINKER_FLAGS
     CMAKE_SHARED_LINKER_FLAGS)
 endif()
 
 if(LLVM_PROFDATA_FILE AND EXISTS ${LLVM_PROFDATA_FILE})
   if ("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang" )
     append("-fprofile-instr-use=\"${LLVM_PROFDATA_FILE}\""