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

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.

Reviewed By: jdoerfert, JonChesterfield

Differential Revision: https://reviews.llvm.org/D95689

Author: shiltian

openmp/cmake/OpenMPTesting.cmake

@@ -58,7 +58,13 @@ if (${OPENMP_STANDALONE_BUILD})
   set(OPENMP_LIT_ARGS "${DEFAULT_LIT_ARGS}" CACHE STRING "Options for lit.")
   separate_arguments(OPENMP_LIT_ARGS)
 else()
-  set(OPENMP_FILECHECK_EXECUTABLE ${LLVM_RUNTIME_OUTPUT_INTDIR}/FileCheck)
+  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()