[clang] [PAC] Define __builtin_ptrauth_type_discriminator (PR #100204)

[Daniil Kovalev via cfe-commits]

https://github.com/kovdan01 commented:

Mostly LGTM, but the mangle-fail test should be fixed before merging - now it just does not look correct (see corresponding comment for details). Otherwise, this looks nice, and I'm OK with merging as soon as the test is fixed (and if nobody else has other concerns)

https://github.com/llvm/llvm-project/pull/100204