c++: lambda in pack expansion using pack in constraint [PR103706]

c++: lambda in pack expansion using pack in constraint [PR103706]

Here when expanding the pack expansion pattern containing a constrained
lambda, the template argument for each Ts is an ARGUMENT_PACK_SELECT,
which we store inside the lambda's LAMBDA_EXPR_REGEN_INFO.  Then during
satisfaction of the lambda's constraint C<Ts> the satisfaction cache
uses this argument as part of the key to the corresponding sat_entry, but
iterative_hash_template_arg and template_args_equal deliberately don't
handle ARGUMENT_PACK_SELECT.

Since it's wrong to preserve ARGUMENT_PACK_SELECT inside a hash table
due to its instability (as documented in iterative_hash_template_arg),
this patch helps make sure the satisfaction cache doesn't see such trees
by resolving ARGUMENT_PACK_SELECT arguments before adding them to
LAMBDA_EXPR_REGEN_INFO.

	PR c++/103706

gcc/cp/ChangeLog:

	* pt.cc (preserve_args): New function.
	(tsubst_lambda_expr): Use it when setting LAMBDA_EXPR_REGEN_INFO.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-lambda19.C: New test.