Add support for ObjC property name to be a single acronym.

Summary:
This change will support cases like:

```
@property(assign, nonatomic) int ID;
```

Reviewers: benhamilton, hokein

Reviewed By: benhamilton

Subscribers: klimek, cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D46374

llvm-svn: 331545