[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