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

[Joe Nash via cfe-commits]

Sisyph wrote:

As far as I know, there is no existing builtin in tree that covers this behavior. The one in https://github.com/llvm/llvm-project/pull/137425 does. If that builtin doesn't land, I can land this without a test. Or perhaps, is there a way to register a 'unit test' builtin?

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