clang-format: Don't break before a case's colon.

Before (with just the right line length:
  switch (a) {
  case some_namespace::some_constant
      :
    return;
  }

After:
  switch (a) {
  case some_namespace::
      some_constant:
    return;
  }

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209725 91177308-0d34-0410-b5e6-96231b3b80d8