[clang-format] fix PR38557 - comments between "default" and ':' causes the...

[clang-format] fix PR38557 - comments between "default" and ':' causes the case label to be treated as an identifier

Summary:
The Bug was reported and fixed by Owen Pan. See the original bug report here: https://bugs.llvm.org/show_bug.cgi?id=38557

Patch by Owen Pan!

Reviewers: krasimir, djasper, klimek

Reviewed By: klimek

Subscribers: JonasToth, cfe-commits

Differential Revision: https://reviews.llvm.org/D50697

llvm-svn: 340624