[clang] Thread Safety Analysis: Don't warn at joins that re-branch on a try-lock result (PR #209796)

[Marco Elver via cfe-commits]

melver wrote:

> Failed to cherry-pick: [f07e2f1](https://github.com/llvm/llvm-project/commit/f07e2f1aabd1f62efd467ed397cb7b26ca60fb6d)
> 
> https://github.com/llvm/llvm-project/actions/runs/30252463558
> 
> Please manually backport the fix and push it to your github fork. Once this is done, please create a [pull request](https://github.com/llvm/llvm-project/compare)

Failed because the release branch didn't include the prior try-lock fix either. Sent https://github.com/llvm/llvm-project/pull/212221

https://github.com/llvm/llvm-project/pull/209796