clang-format: Improve line breaks in @property.

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

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

llvm-svn: 187577