clang-format: Don't allow labels when expecting declarations.

clang-format: Don't allow labels when expecting declarations.

This fixes formatting unnamed bitfields (llvm.org/PR21999).

Before:
  struct MyStruct {
    uchar data;
  uchar:
    8;
  uchar:
    8;
    uchar other;
  };

After:
  struct MyStruct {
    uchar data;
    uchar : 8;
    uchar : 8;
    uchar other;
  };

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@234318 91177308-0d34-0410-b5e6-96231b3b80d8