[clang] Add `pragma clang scope [push|pop]` (PR #121025)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> I wonder if we had an rfc for this. Maybe we should solicit some feedback before moving forward with this extension.

Agreed, I think this should have an RFC for wider discussion.

https://github.com/llvm/llvm-project/pull/121025