c++: handle misspelled concepts and missing #include <concepts>

gcc/cp/ChangeLog:
	* name-lookup.cc (suggest_alternative_in_explicit_scope):
	Gracefully handle non-namespaces, such as scoped enums.
	* parser.cc (cp_parser_name_lookup_error): Provide
	a name_hint for the case where we're in an explicit scope.
	* std-name-hint.gperf: Add <concepts>.
	* std-name-hint.h: Regenerate.

gcc/testsuite/ChangeLog:
	* g++.dg/concepts/missing-header.C: New test.
	* g++.dg/concepts/misspelled-concept.C: New test.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>