[NFC][clang][PCH][ObjC] Add some missing `VisitStmt(S);`

Summary:
These ObjC AST classes inherit from Stmt, but don't call `VisitStmt(S);`.
Some were founded with help of existing tests (with `NumStmtFields` bumped to `1`),
but some of them don't even have PCH test coverage. :/

Reviewers: arphaman, sammccall, smeenai, aprantl, rsmith, jordan_rose

Reviewed By: jordan_rose

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59197

llvm-svn: 355987