[clang] [Clang] Introduce scoped variants of GNU atomic functions (PR #72280)

[Jon Chesterfield via cfe-commits]

JonChesterfield wrote:

Looks solid to me. The patch to clang is long but straightforward and the tests look reassuringly exhaustive. Probably good that you ignored my name suggestion of integers 0 through N.

This patch is partly motivated by us wanting device scope atomics in libc. It removes one of the remaining stumbling blocks for people who like freestanding C++ as a GPU programming language.

Hopefully the clang people consider the extension acceptable.

https://github.com/llvm/llvm-project/pull/72280