c++: defer all consteval in default args [DR2631]

c++: defer all consteval in default args [DR2631]

The proposed resolution of CWG2631 extends our current handling of
source_location::current to all consteval functions: default arguments
are not evaluated until they're used in a call, the same should apply to
evaluation of immediate invocations.  And similarly for default member
initializers.

Previously we folded source_location::current in cp_fold_r; now we fold all
consteval calls in default arguments/member initializers in bot_replace.

	DR 2631

gcc/cp/ChangeLog:

	* cp-tree.h (source_location_current_p): Remove.
	* name-lookup.h (struct cp_binding_level): Remove
	immediate_fn_ctx_p.
	* call.cc (in_immediate_context): All default args
	and DMI are potentially immediate context.
	(immediate_invocation_p): Don't treat source_location specially.
	(struct in_consteval_if_p_temp_override): Move to cp-tree.h.
	* constexpr.cc (get_nth_callarg): Move to cp-tree.h.
	* cp-gimplify.cc (cp_fold_r): Don't fold consteval.
	* name-lookup.cc (begin_scope): Don't set immediate_fn_ctx_p.
	* parser.cc (cp_parser_lambda_declarator_opt): Likewise.
	(cp_parser_direct_declarator): Likewise.
	* pt.cc (tsubst_default_argument): Open sk_function_parms level.
	* tree.cc (source_location_current_p): Remove.
	(bot_replace): Fold consteval here.
	(break_out_target_exprs): Handle errors.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/consteval-defarg3.C: New test.