clang-format: Don't merge simple blocks in case statements.

Before:
  switch (a) {
  case 1: { return 'a'; }
  }

After:
  switch (a) {
  case 1: {
    return 'a';
  }
  }

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205611 91177308-0d34-0410-b5e6-96231b3b80d8