c++: lambda in pack expansion [PR115378]

c++: lambda in pack expansion [PR115378]

Here find_parameter_packs_r is incorrectly treating the 'auto' return
type of a lambda as a parameter pack due to Concepts-TS specific logic
added in r6-4517, leading to confusion later when expanding the pattern.

Since we intend on removing Concepts TS support soon anyway, this patch
fixes this by restricting the problematic logic with flag_concepts_ts.
Doing so revealed that add_capture was relying on this logic to set
TEMPLATE_TYPE_PARAMETER_PACK for the 'auto' type of an pack expansion
init-capture, which we now need to do explicitly.

	PR c++/115378

gcc/cp/ChangeLog:

	* lambda.cc (lambda_capture_field_type): Set
	TEMPLATE_TYPE_PARAMETER_PACK on the auto type of an init-capture
	pack expansion.
	* pt.cc (find_parameter_packs_r) <case TEMPLATE_TYPE_PARM>:
	Restrict TEMPLATE_TYPE_PARAMETER_PACK promotion with
	flag_concepts_ts.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/decltype-auto-103497.C: Adjust expected diagnostic.
	* g++.dg/template/pr95672.C: Likewise.
	* g++.dg/cpp2a/lambda-targ5.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>