[clang] clang: Allow targets to set custom metadata on atomics (PR #96906)

[Matt Arsenault via cfe-commits]

arsenm wrote:

* **#96906** <a href="https://app.graphite.dev/github/pr/llvm/llvm-project/96906?utm_source=stack-comment-icon" target="_blank"><img src="https://static.graphite.dev/graphite-32x32-black.png" alt="Graphite" width="10px" height="10px"/></a> 👈
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https://github.com/llvm/llvm-project/pull/96906