[clang] clang/AMDGPU: Emit atomicrmw from {global|flat}_atomic_fadd_v2f16 builtins (PR #96873)

[Matt Arsenault via cfe-commits]

arsenm wrote:

### Merge activity

* **Aug 20, 2:53 PM EDT**: @arsenm started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/96873).


https://github.com/llvm/llvm-project/pull/96873