Clean up ObjCPropertyDecl printing

Summary:
- `@property(attr, attr2)` instead of `@property ( attr,attr2 )`.
- Change priority of attributes (see code/comments inline).
- Support for printing weak and unsafe_unretained attributes.

Subscribers: arphaman, jfb, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57965

llvm-svn: 357937