Thread Safety: also look at ObjC methods

Summary:
SExprBuilder::translateDeclRefExpr was only looking at FunctionDecl and not also looking at ObjCMethodDecl. It should consider both because the attributes can be used on Objective-C as well.

<rdar://problem/48941331>

Reviewers: dexonsmith, erik.pilkington

Subscribers: jkorous, jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59523

llvm-svn: 356940