[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

llvm-svn: 358898