[Format] Add debugging to ObjC language guesser

Summary:
To handle diagnosing bugs where ObjCHeaderStyleGuesser guesses
wrong, this diff adds a bit more debug logging to the Objective-C
language guesser.

Reviewers: krasimir

Reviewed By: krasimir

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D54110

llvm-svn: 346144