Summary:
Add the `objcImplementationDecl` matcher. See related: D30854

Tested with:

```
./tools/clang/unittests/ASTMatchers/ASTMatchersTests
```

Reviewers: aaron.ballman, compnerd, alexshap

Reviewed By: aaron.ballman

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D37643

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@312889 91177308-0d34-0410-b5e6-96231b3b80d8