This patch implements generation of remaining header search arguments.
It's done manually in C++ as opposed to TableGen, because we need the flexibility and don't anticipate reuse.

This patch also tests the generation of header search options via a round-trip. This way, the code gets exercised whenever Clang is built and tested in asserts mode. All `check-clang` tests pass.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D94472