[cmake] Include LLVMTestingSupport when doing stand-alone build

[mgorny]

Summary

Explicitly include and build lib/Testing/Support from LLVM sources when
doing a stand-alone build. This is necessary since clangd tests started
to depend on LLVMTestingSupport library which is neither installed
by LLVM, nor built by clang itself.

Since completely separate build of clang-tools-extra is not supported,
this relies on variables set by clang CMakeLists.

[ilya-biryukov]

This LG to fix the standalone build.
However, why not install LLVMTestingSupport in llvm? It feels that we should either include it or disable the tests for standalone builds. And disabling the tests seems wrong.
WDYT?
@sammccall, in case he also has an opinion on this.

[sammccall]

This seems fine to me too to fix the immediate issue.

Personally I'd rather drop our usage of llvmtestingsupport as it provides marginal value anyway. But also happy to do this as a cleanup afterwards.

Installing it in llvm seems like a good idea to me too, but outside my area.

[mgorny]

I've based this change on an earlier fix to lldb when that project started using LLVMTestingSupport.

That change in turn was based on how we handle gtest across the projects. As for gtest, I think installing was not considered as an option since different projects may build it with different flags. However, I did not investigate it in detail.

I don't specifically mind installing LLVMTestingSupport in any way. This solution was simply the least effort way of solving the immediate problem, given that we need LLVM sources for gtest anyway.

clang-tools-extra/trunk/unittests/CMakeLists.txt

 add_custom_target(ExtraToolsUnitTests)
 set_target_properties(ExtraToolsUnitTests PROPERTIES FOLDER "Extra Tools Unit Tests")
 
 function(add_extra_unittest test_dirname)
   add_unittest(ExtraToolsUnitTests ${test_dirname} ${ARGN})
 endfunction()
 
+if(CLANG_BUILT_STANDALONE)
+  # LLVMTestingSupport library is needed for clangd tests.
+  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_subdirectory(change-namespace)
 add_subdirectory(clang-apply-replacements)
 add_subdirectory(clang-move)
 add_subdirectory(clang-query)
 if(CLANG_ENABLE_STATIC_ANALYZER)
   add_subdirectory(clang-tidy)
 endif()
 add_subdirectory(clangd)
 add_subdirectory(include-fixer)