clang-format: Improve line breaks in @property.

Before:
  @property(nonatomic, assign,
            readonly) NSString *looooooooooooooooooooooooooooongName;

After:
  @property(nonatomic, assign, readonly)
      NSString *looooooooooooooooooooooooooooongName;

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@187577 91177308-0d34-0410-b5e6-96231b3b80d8