[clang] [PAC] Add support for __ptrauth type qualifier (PR #100830)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> @rjmccall sent an RFC for upstreaming pointer authentication work to llvm-dev in 2019, which mentions the `__ptrauth` qualifier. https://discourse.llvm.org/t/rfc-pointer-authentication-for-arm64e/53433
> 
> I don't think we sent an RFC for adding support for the `__ptrauth` qualifier.
>
> I'll try to come up with an RFC for the qualifier today.

Thank you for working on the RFC -- there really wasn't much community engagement on the post in 2019, so reminding the community about the original RFC would also be really helpful. This is adding a fairly significant amount of code to support the extension (not just the qualifier, but the PAC work in general), so I want to make sure the community is comfortable with it.

(FWIW, I have no opinions about it yet because I'm still learning what it is.)

https://github.com/llvm/llvm-project/pull/100830