Remove faulty assertion in llvm-pdbutil

If a class's first data member is an instance of an empty class, then an
assertion in the PrettyClassLayoutGraphicalDumper would fail. The
storage is reserved, but it's not marked as in use.

As far as I understand, it's the assertion that's faulty, so I removed it
and updated the nearby comment.

Found by running llvm-pdbutil against its own PDB, and this assertion would
fail on HashAdjusters, which is a HashTable whose first data member is a
TraitsT, which is a PdbHashTraits<T>, which is an empty struct. (The struct
has a specialization for uint32_t, but that specialization doesn't apply
here because the T is actually ulittle32_t.)

Differential Revision: https://reviews.llvm.org/D45645

llvm-svn: 330135