[OpenMP] Disable tests if FileCheck is not available in in-tree building

[tianshilei1992]

Summary

FileCheck is required for OpenMP tests. The current detection can fail
if building OpenMP in-tree when user sets LLVM_INSTALL_TOOLCHAIN_ONLY=ON. As a
result, CMake will raise an error and the compilation will be broken. This patch
fixed the issue. When FileCheck is not a target, tests will just be skipped.

[jdoerfert]

LG

[JonChesterfield]

I think filecheck is sometimes installed from a distribution instead of using the one from llvm, but am not totally sure that's a use case worth supporting.

[tianshilei1992]

In D95689#2533584, @JonChesterfield wrote:

I think filecheck is sometimes installed from a distribution instead of using the one from llvm, but am not totally sure that's a use case worth supporting.

Actually it's not worth supporting because some features might not be supported by the old version from distribution. I did experience the issue once. :-(

openmp/cmake/OpenMPTesting.cmake

if (${OPENMP_STANDALONE_BUILD})
   # Set lit arguments.
   set(DEFAULT_LIT_ARGS "-sv --show-unsupported --show-xfail")
   if (MSVC OR XCODE)
     set(DEFAULT_LIT_ARGS "${DEFAULT_LIT_ARGS} --no-progress-bar")
   endif()
   set(OPENMP_LIT_ARGS "${DEFAULT_LIT_ARGS}" CACHE STRING "Options for lit.")
   separate_arguments(OPENMP_LIT_ARGS)
 else()
+  if (NOT TARGET "FileCheck")
+    message(STATUS "Cannot find 'FileCheck'.")
+    message(WARNING "The check targets will not be available!")
+    set(ENABLE_CHECK_TARGETS FALSE)
+  else()
     set(OPENMP_FILECHECK_EXECUTABLE ${LLVM_RUNTIME_OUTPUT_INTDIR}/FileCheck)
+  endif()
   set(OPENMP_NOT_EXECUTABLE ${LLVM_RUNTIME_OUTPUT_INTDIR}/not)
 endif()
 
 # Macro to extract information about compiler from file. (no own scope)
 macro(extract_test_compiler_information lang file)
   file(READ ${file} information)
   list(GET information 0 path)
   list(GET information 1 id)