[clang] [clang] Wire -fptrauth-returns to "ptrauth-returns" fn attribute. (PR #102416)

[Ahmed Bougacha via cfe-commits]

ahmedbougacha wrote:

> @ahmedbougacha Non-blocking nit: you might want to add a test in clang/test/CodeGen/ptrauth-function-attributes.c (like you have for auth traps in https://github.com/llvm/llvm-project/pull/102417)

Oh yeah that's certainly required;  I think I dropped it accidentally when rebasing onto the driver-only part of fptrauth-returns

https://github.com/llvm/llvm-project/pull/102416