clang-format: Fix bug in ObjC method declaration formatting.

Also disallow breaking between "@" and "{" or "[".

Before:
  - (NSAttributedString *)attributedStringForSegment:(NSUInteger)segment
                                               index:(NSUInteger)index
                                          attributes:(NSDictionary *)attributes
                                  nonDigitAttributes:(NSDictionary *)
      nonDigitAttributes;
  [mailComposeViewController
     setToRecipients:@
     [ NSBundle.mainBundle.infoDictionary[@"ABBFeedbackEmail"] ]];

After:
  - (NSAttributedString *)attributedStringForSegment:(NSUInteger)segment
                                               index:(NSUInteger)index
                                          attributes:(NSDictionary *)attributes
                                  nonDigitAttributes:
                                      (NSDictionary *)nonDigitAttributes;
  [mailComposeViewController
      setToRecipients:
          @[ NSBundle.mainBundle.infoDictionary[@"ABBFeedbackEmail"] ]];

This fixes llvm.org/PR18030.