User-controllable diagnostics for Clang frontend plugins

[AaronBallman]

RFC: https://discourse.llvm.org/t/rfc-user-controllable-diagnostics-for-clang-frontend-plugins/91266

[AaronBallman]

The Clang Area Team discussed this RFC at the meeting on Jul 30, note can be found at: https://docs.google.com/document/d/13-68BCOt8kH5k_56fURBi7u7Ap97Z0EshgkJXDudbJM/edit?pli=1&tab=t.0

We think there's general support for the idea but we don't feel comfortable accepting the RFC just yet and would rather see more bake time and discussion. Aaron posted a comment to the RFC here: https://discourse.llvm.org/t/rfc-user-controllable-diagnostics-for-clang-frontend-plugins/91266/15