This allows -DLLVM_ENABLE_RUNTIMES=libcxxabi to work.
This functionality was removed in D125561 (among other such removals) as
dead code, because it was only available as part of the standalone build
The functionality as added back as it was, except target_include_directories is used instead of the cruder target_compile_options (with an MSVC vs not MSVC conditional split). That is just wholly better.