If the infcx has observed other errors, then suppress both default type
parameter fallback (which can be unreliable, as the full constraint set
is not available) and errors related to unresovled
variables (annoyingly, integer type variables cannot currently be
unified with error, so that has to be a separate mechanism). Also add a
flag to `infcx` to allow us to independently indicate when we have
observed an error and hence should trigger this suppression mode.

It is odd to have this logic strewn about.  This also means that all
calls to `type_is_known_to_be_sized` are encapsulated in the
cast code, in case we want to update that logic.

Do not require the target type to be fully known,
either. This allows code like `let x: *const () = 0 as _` to work
(see regression test).