Fix Bug 38713: clang-format mishandles a short block after "default:" in a switch statement

Summary:
See https://bugs.llvm.org/show_bug.cgi?id=38713

Patch by Owen Pan!

Reviewers: djasper, klimek, sammccall

Reviewed By: sammccall

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D51294

llvm-svn: 341284