[llvm] [TySan] Skip instrumentation for function declarations (PR #122488)

[Florian Hahn via llvm-commits]

https://github.com/fhahn approved this pull request.

LGTM, thanks.

Please add `Fixes https://github.com/llvm/llvm-project/issues/122467` to the PR description before merging, so the issue will auto-close

https://github.com/llvm/llvm-project/pull/122488