Add objcCategoryImplDecl matcher

Summary:
Add `objcCategoryImplDecl` which matches ObjC category definitions
(`@implementation`). This matcher complements `objcCategoryDecl` (`@interface`)
which was added in D30854.

Reviewers: aaron.ballman, malcolm.parsons, alexshap

Reviewed By: aaron.ballman

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D39293

llvm-svn: 316670