[clang] Fix linking to LLVMTestingAnnotations in standalone build

The LLVMTestingAnnotations library that is now used by unittests
is not installed as part of LLVM.  In order to make it possible to build
unittests when performing the standalone build of clang, build
the library from LLVM sources locally.  This mirrors the existing logic
for LLVMTestingSupport.

Differential Revision: https://reviews.llvm.org/D142449