c++: requires-expression and tentative parse [PR94480]

c++: requires-expression and tentative parse [PR94480]

The problem here was that cp_parser_requires_expression committing to a
tentative parse confused cp_parser_decltype_expr, which needs to still be
tentative.  The only reason to commit here is to get syntax errors within
the requires-expression, which we can still do when the commit is firewalled
from the enclosing context.

gcc/cp/ChangeLog
2020-04-07  Jason Merrill  <jason@redhat.com>

	PR c++/94480
	* parser.c (cp_parser_requires_expression): Use tentative_firewall.