Thread safety analysis: Handle conditional expression in getTrylockCallExpr

Thread safety analysis: Handle conditional expression in getTrylockCallExpr

Summary:
We unwrap conditional expressions containing try-lock functions.

Additionally we don't acquire on conditional expression branches, since
that is usually not helpful. When joining the branches we would almost
certainly get a warning then.

Hopefully fixes an issue that was raised in D52398.

Reviewers: aaron.ballman, delesley, hokein

Reviewed By: aaron.ballman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D52888

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@343902 91177308-0d34-0410-b5e6-96231b3b80d8