c++: constrained auto in lambda using outer tparms [PR103706]

c++: constrained auto in lambda using outer tparms [PR103706]

Here we're crashing during satisfaction of the lambda's placeholder type
constraints because the constraints depend on the template arguments
from the enclosing scope, which aren't part of the lambda's DECL_TI_ARGS.

This patch fixes this by making do_auto_deduction consider the
"regenerating" template arguments of a lambda for satisfaction,
mirroring what's done in satisfy_declaration_constraints.

	PR c++/103706

gcc/cp/ChangeLog:

	* constraint.cc (satisfy_declaration_constraints): Use
	lambda_regenerating_args instead.
	* cp-tree.h (lambda_regenerating_args): Declare.
	* pt.c (lambda_regenerating_args): Define, split out from
	satisfy_declaration_constraints.
	(do_auto_deduction): Use lambda_regenerating_args to obtain the
	full set of outer template arguments for satisfaction when
	inside a lambda.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-lambda18.C: New test.

(cherry picked from commit 34ba3d9a2bf72742b1c150a2dd17d10e3e3f0964)