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.