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

[Jon Chesterfield via cfe-commits]

JonChesterfield wrote:

The patch to SemaExpr looks reasonable to me. I'd suggest that goes in separate from the amdgpu intrinsic stuff.

I'd test this by tweaking the code to do the current lowering _and_ the proposed and check that they do exactly the same thing on all the existing builtins, then drop the current code path, but ymmv in terms of how that strategy interacts with our code review system.

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