[clang] [unittest] Import LLVMTestingSupport if necessary

[mgorny]

Summary

Add LLVMTestingSupport directory from LLVM_MAIN_SRC_DIR when building
clang stand-alone and LLVMTestingSupport library is not present. This
is needed to fix stand-alone builds without clang-tools-extra.

// The code is copied verbatim from clangd unittests

[gribozavr]

Thanks for the fix! I'm not a fan of the complexity that standalone builds create, but if we are supporting them... oh well.

[mgorny]

Thanks. Could you also ack the backport in https://bugs.llvm.org/show_bug.cgi?id=43281?

cfe/trunk/unittests/CMakeLists.txt

 add_custom_target(ClangUnitTests)
 set_target_properties(ClangUnitTests PROPERTIES FOLDER "Clang tests")
 
+if(CLANG_BUILT_STANDALONE)
+  # LLVMTestingSupport library is needed for some of the unittests.
+  if (EXISTS ${LLVM_MAIN_SRC_DIR}/lib/Testing/Support
+      AND NOT TARGET LLVMTestingSupport)
+    add_subdirectory(${LLVM_MAIN_SRC_DIR}/lib/Testing/Support
+      lib/Testing/Support)
+  endif()
+endif()
+
 # add_clang_unittest(test_dirname file1.cpp file2.cpp)
 #
 # Will compile the list of files together and link against the clang
 # Produces a binary named 'basename(test_dirname)'.
 function(add_clang_unittest test_dirname)
   add_unittest(ClangUnitTests ${test_dirname} ${ARGN})
 endfunction()