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

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@340624 91177308-0d34-0410-b5e6-96231b3b80d8