[clang] [clang] Fix thread_local incorrectly treated as tentative definition (PR #219780)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

The changes should come with a release note in `clang/docs/ReleaseNotes.md` so users know about the fix, but otherwise this LGTM

https://github.com/llvm/llvm-project/pull/219780