[clang] [Sema] Fix bug in builtin AS override (PR #138141)

[Krzysztof Drewniak via cfe-commits]

krzysz00 wrote:

Ok, so this is a pure bugfix

Since this bit of infrastructure's kinda underdocumented, why did the `"t"` flag need to be removed and what does it mean?

https://github.com/llvm/llvm-project/pull/138141