[gn build] Add build files for clang/tools/{c-arcmt-test,c-index-test} and...

[gn build] Add build files for clang/tools/{c-arcmt-test,c-index-test} and their dependency clang/tools/libclang

libclang is somewhat incomplete. It's just enough to get check-clang to pass,
but that requires it to be pretty complete. The biggest thing is that it's not
built as a shared library on Linux. The libclang/BUILD.gn file has a comment
with details on what else is missing.

Differential Revision: https://reviews.llvm.org/D56059

llvm-svn: 350054