Replace two fallthrough annotations after covered switch with unreachable

Both preceding switches handle all possible enumerators, so the
fallthrough is actually unreachable. This strengthens that to an
assertion.

The first instance had a comment from 2010 indicating that fallthrough
was possible, but that was back when we had a unary operator for
offsetof. Now it is its own expression kind, so the annotation was
stale.

llvm-svn: 345862