PR c++/92105 - decltype(decltype) error cascade.

The primary change here is to do the CPP_DECLTYPE replacement even when we
get an error, so we don't keep trying and giving the same parse error each
time.  We also commit to the tentative firewall parse more often, leading to
better diagnostics.

	* parser.c (cp_parser_decltype_expr): Don't tentative_firewall here.
	(cp_parser_decltype): Do it here.  Remember a non-tentative error.

From-SVN: r279237