[clang] [C] Add -Wtentative-definition-compat (PR #137967)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Is there maybe a quick fix or should we add a revert (to revert to green)?

I'm still investigating, so unclear on a quick fix. But to make sure I understand correctly, things *are* green (no bots are failing, right?), this is just a false positive? Or am I missing something about the severity?

https://github.com/llvm/llvm-project/pull/137967