[clang] [llvm] [clang] Add `-verify-directives` cc1 flag (PR #179835)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

Patch is pretty sizable so I wasn't able to do a thorough review (particularly in VerifyDiagnosticsConsumer.cpp), but hopefully someone else can take a look.  I have some suggestions/etc with docs.

https://github.com/llvm/llvm-project/pull/179835