[sema][objc] Minor refactor to OverrideSearch. NFCI.
Summary:
- Removed a member that was only used during construction.
- Use range-based for iteration when accessing the result of the search.
- Require an ObjCMethodDecl reference upon construction of an
- Constify.
Reviewers: rjmccall
Reviewed By: rjmccall
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D60850