[DebugInfo] Ensure complete type is emitted with -fstandalone-debug

The logic for upgrading a class from a forward decl to a complete type
was not checking the debug info emission level before applying the
vtable optimization. This meant we ended up without debug info for a
class which was required to be complete. I noticed it because it
triggered an assertion during CodeView emission, but that's a separate
issue.

llvm-svn: 281057