[clang] [PAuth] Use different discriminators for __int128_t / __uint128_t / _BitInt(n) (PR #140276)

[Anton Korobeynikov via cfe-commits]

asl wrote:

@ojhunt Rebased this PR. Also, the flag now is opt-in, not enabled by default (same as for other signing schema bits, e.g. for typeinfo type discrimination, etc.).

https://github.com/llvm/llvm-project/pull/140276