Thread safety analysis: Add note for double unlock

Summary:
When getting a warning that we release a capability that isn't held it's
sometimes not clear why. So just like we do for double locking, we add a
note on the previous release operation, which marks the point since when
the capability isn't held any longer.

We can find this previous release operation by looking up the
corresponding negative capability.

Reviewers: aaron.ballman, delesley

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D81352