Thread safety analysis: Improve diagnostics for double locking

Summary:
We use the existing diag::note_locked_here to tell the user where we saw
the first locking.

Reviewers: aaron.ballman, delesley

Reviewed By: aaron.ballman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D56967

llvm-svn: 352549