[clang] [clang][PAC] ptrauth_qualifier and ptrauth_intrinsic should only be available on Darwin (PR #153912)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

I realized that this can't be made to work nicely and automatically no matter how much I would like it to as of course the macro based version of `__has_ptrauth_feature` can't check for a macro being defined in expression contexts. Oh well, I guess that dream is over. I'll just stick to the `__PTRAUTH__` define and resign myself to the feature checks for things other than the core intrinsic and qualifier tests. Boo :(

https://github.com/llvm/llvm-project/pull/153912