Revert "c++: delayed parse DMI [PR96645]"

The breakage from r12-7804 (in WebKit, particularly) is more of a can of
worms than I think we can address in GCC 12, so let's return to the GCC 11
status quo for now and try again in stage 1.

I think the change was correct for the current standard, but the standard
needs a fix in this area; this is CWG issue 2335.

	PR c++/96645

This reverts commits r12-7804 and r12-7929.