MSVCCompat: Don't produce an invalid AST when accepting void pseudo-dtors

We accept 'void *p; p->~void();' for MSVC compatibility since r148682.
However, we were returning ExprError, rather than producing an AST,
despite only diagnosing it with a warning.  CodeGen noticed that the
template function specialization had an invalid AST, and therefore
didn't generate code for it.  This change makes us produce an AST with a
void pseudo-dtor call.

Part of PR18256.