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;
  };

llvm-svn: 234318