c++: Use a TEMPLATE_INFO to hold regenerated-lambda info

c++: Use a TEMPLATE_INFO to hold regenerated-lambda info

A TEMPLATE_INFO is a natural fit for what LAMBDA_EXPR_REGENERATED_FROM
and LAMBDA_EXPR_REGENERATING_TARGS hold, so let's use it instead.

gcc/cp/ChangeLog:

	* cp-tree.h (LAMBDA_EXPR_REGENERATED_FROM)
	(LAMBDA_EXPR_REGENERATING_TARGS): Replace these with ...
	(LAMBDA_EXPR_REGEN_INFO): ... this.
	(tree_lambda_expr::regenerated_from)
	(tree_lambda_expr::regenerating_targs): Replace these with ...
	(tree_lambda_expr::regen_info): ... this.
	* constraint.cc (satisfy_declaration_constraints): Adjust
	accordingly.
	* lambda.c (build_lambda_expr): Likewise.
	* pt.c (regenerated_lambda_fn_p): Likewise.
	(most_general_lambda): Likewise.
	(tsubst_lambda_expr): Likewise.