Summary:
Previously, the assert_capability attribute was completely ignored by
thread safety analysis.

Reviewers: delesley, rnk

Reviewed By: delesley

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D36122

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@309725 91177308-0d34-0410-b5e6-96231b3b80d8