[PR c++/84789] adjust testcase for -fconcepts

[PR c++/84789] adjust testcase for -fconcepts

When compiling with -fconcepts,
cp_parser_template_declaration_after_export calls
cp_parser_template_introduction and that preparses qualified-ids not
preceded by typename in such a way that, when we get to
cp_parser_parse_and_diagnose_invalid_type_name and then
cp_parser_diagnose_invalid_type_name, the nested name specifier no
longer carries the previous template-dependent context, so we don't
stand a chance to suggest the use of 'typename' any more.  Thus,
tolerate in the testcase the poorer error messages we get.

for  gcc/testsuite/ChangeLog

	PR c++/84789
	* g++.dg/template/pr84789.C: Adjust for testing with
	-fconcepts too.

From-SVN: r258915