[ASTImporter] Add test for ObjCAtTryStmt/ObjCAtCatchStmt/ObjCAtThrowStmt

Reviewers: a.sidorin, a_sidorin

Reviewed By: a_sidorin

Subscribers: martong, cfe-commits

Differential Revision: https://reviews.llvm.org/D51121

llvm-svn: 340468