c++: elaborated-type-spec in requires-expr [PR101677]

c++: elaborated-type-spec in requires-expr [PR101677]

We were failing to declare class S in the global namespace because we were
treating the requires-expression parameter scope as a normal block scope, so
the implicit declaration went there.

It seems to me that the requires parameter scope is more like a function
parameter scope (not least in the use of the word "parameter"), so let's
change the scope kind.  But then we need to adjust the prohibition on
placeholders declaring implicit template parameters.

	PR c++/101677

gcc/cp/ChangeLog:

	* name-lookup.h (struct cp_binding_level): Add requires_expression
	bit-field.
	* parser.cc (cp_parser_requires_expression): Set it.
	(synthesize_implicit_template_parm): Check it.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-pr67178.C: Adjust error.
	* g++.dg/cpp2a/concepts-requires28.C: New test.