[clang] [C23] Accept an _Atomic underlying type (PR #147802)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> I THINK this is right, but we also need to have a codegen test to make sure we don't try to do goofiness with these.

I was thinking the ast-dump test sufficed because it shows that we set the underlying type to non-atomic in the AST. But I can certainly add a codegen test, do you have anything in particular in mind?

https://github.com/llvm/llvm-project/pull/147802