Fix TBAA representation for zero-sized fields and unnamed bit-fields.

Unnamed bit-fields should not be represented in the TBAA metadata
because they do not represent storage fields (they only affect layout).

Zero-sized fields should not be represented in the TBAA metadata
because by definition they have no associated storage (so we will never
emit a load or store through them), and they might not appear in
declaration order within the struct layout.

Fixes a verifier failure when emitting a TBAA-enabled load through a
class type containing a zero-sized field.

llvm-svn: 364140