Formatter: Detect ObjC array literals.

Use this to add a space after "@[" and before "]" for now.

Later, I want to use this to format multi-line array literals nicer, too.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174822 91177308-0d34-0410-b5e6-96231b3b80d8