Formatter: Add support for @interface.

Previously:
@interface Foo + (id)init; @end

Now:
@interface Foo
+ (id)init;
@end

Some tweaking remains, but this is a good first step.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171995 91177308-0d34-0410-b5e6-96231b3b80d8