Thread safety analysis: Unwrap __builtin_expect in getTrylockCallExpr

Summary:
When people are really sure they'll get the lock they sometimes use
__builtin_expect. It's also used by some assertion implementations.
Asserting that try-lock succeeded is basically the same as asserting
that the lock is not held by anyone else (and acquiring it).

Reviewers: aaron.ballman, delesley

Reviewed By: aaron.ballman

Subscribers: kristina, cfe-commits

Differential Revision: https://reviews.llvm.org/D52398

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@343681 91177308-0d34-0410-b5e6-96231b3b80d8