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

[Akira Hatanaka via cfe-commits]

ahatanak 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.

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