Make isDefinition matcher support ObjCMethodDecl

Summary:
Allow the `isDefinition()` matcher to apply to `ObjCMethodDecl` nodes, in
addition to those it already supports. For whatever reason, `ObjCMethodDecl`
does not inherit from `FunctionDecl` and so this is specialization is necessary.

Reviewers: aaron.ballman, malcolm.parsons, alexshap

Reviewed By: aaron.ballman

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D39948

llvm-svn: 318152