Correctly handle zero-sized but non-empty base classes in IRGen.

Fixes rdar://20621065.

A more elegant fix would preclude this case by defining the
rules such that zero-size classes are always formally empty.
I believe the only extensions which create zero-size classes
right now are flexible arrays and zero-length arrays; it's
not abstractly unreasonable to say that those don't count
as members for the purposes of emptiness, just as zero-width
bitfields don't count.  But that's an ABI-affecting change
and requires further discussion; in the meantime, let's not
assert / miscompile.

llvm-svn: 235815