c++: Delay normalizing nested requirements until satisfaction

This sets up the functionality for controlling the initial set of
template parameters to pass to normalization when dealing with a
constraint-expression that is not associated with some constrained
declaration, for instance when normalizing a nested requirement of a
requires expression, or the constraints on a placeholder type.

The main new ingredient here is the data member norm_info::initial_parms
which can be set by callers of the normalization routines to communicate
the in-scope template parameters for the supplied constraint-expression,
rather than always falling back to using current_template_parms.

This patch then uses this functionality in our handling of nested
requirements so that we can delay normalizing them until needed for
satisfaction.  We currently immediately normalize nested requirements at
parse time, where we have the necessary template context, and cache the
normal form in their TREE_TYPE node.  With this patch, we now delay
normalization until needed (as with other constraint expressions), and
instead store the current value of current_template_parms in their
TREE_TYPE node (which we use to restore the template context at
normalization time).

In the subsequent patch, this functionality will also be used to
normalize placeholder type constraints during auto deduction.

gcc/cp/ChangeLog:

	* constraint.cc (build_parameter_mapping): Rely on the caller to
	determine the in-scope template parameters.
	(norm_info::norm_info): Delegate the tsubst_flags_t constructor
	to the two-parameter constructor.  In the two-parameter
	constructor, fold in the definition of make_context, set
	initial_parms appropriately, and don't set the now-removed
	orig_decl member.
	(norm_info::make_context): Remove, now that its only use is
	inlined into the caller.
	(norm_info::update_context): Adjust call to
	build_parameter_mapping to pass in the relevant set of in-scope
	template parameters.
	(norm_info::ctx_parms): Define this member function.
	(norm_info::context): Initialize to NULL_TREE.
	(norm_info::orig_decl): Remove this data member.
	(norm_info::initial_parms): Define this data member.
	(normalize_atom): Adjust call to build_parameter_mapping to pass
	in the relevant set of in-scope template parameters.  Use
	info.initial_parms instead of info.orig_decl.
	(normalize_constraint_expression): Take a norm_info object
	instead of a bool.  Cache the result of normalization.
	(tsubst_nested_requirement): Call satisfy_constraint_expression
	instead of satisfy_constraint, so that we normalize on demand.
	(satisfy_constraint_expression): Handle a NESTED_REQ argument.
	Adjust call to normalize_constraint_expression.
	(finish_nested_requirement): Set the TREE_TYPE of the NESTED_REQ
	to current_template_parms.
	(diagnose_nested_requirements): Go through
	satisfy_constraint_expression, as with tsubst_nested_requirement.

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