Thread safety analysis: Handle ObjCIvarRefExpr in SExprBuilder::translate

Summary:
This imitates the code for MemberExpr.

Fixes PR38896.

Reviewers: aaron.ballman, delesley, lukasza, rjmccall

Reviewed By: delesley

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D52200

llvm-svn: 342600