[cmake] Include LLVMTestingSupport when doing stand-alone build

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.

Differential Revision: https://reviews.llvm.org/D45409

llvm-svn: 329594