Formatter: Add a test for bitfields.

They work fine, but this fifth use of colons (after labels, in ?:,
in initalizer lists in constructors, in objc method expressions, and in
bitfields) wasn't covered by tests yet.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172377 91177308-0d34-0410-b5e6-96231b3b80d8