c++: Fix late-parsed default arg context

c++: Fix late-parsed default arg context

Jakub noticed that we weren't recognizing a default argument for a consteval
member function as being in immediate function context because there was no
function parameter scope to look at.

Note that this patch doesn't actually push the parameters into the scope,
that happens in a separate commit.

gcc/cp/ChangeLog:

	* name-lookup.c (begin_scope): Set immediate_fn_ctx_p.
	* parser.c (cp_parser_late_parsing_default_args): Push
	sk_function_parms scope.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/consteval-defarg1.C: New test.