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

llvm-svn: 343902