c++: Non-type-dependent variadic lambda init-capture [PR94483]

In this PR, we're ICEing on a use of an 'int... a' template parameter pack as
part of the variadic lambda init-capture [...z=a].

The unexpected thing about this variadic init-capture is that it is not
type-dependent, and so the call to do_auto_deduction from
lambda_capture_field_type actually resolves its type to 'int' instead of exiting
early like it does for a type-dependent variadic initializer.  This later
confuses add_capture which, according to one of its comments, assumes that
'type' is always 'auto' for a variadic init-capture.

The simplest fix (and the approach that this patch takes) seems to be to avoid
doing auto deduction in lambda_capture_field_type when the initializer uses
parameter packs, so that we always return 'auto' even in the non-type-dependent
case.

gcc/cp/ChangeLog:

	PR c++/94483
	* lambda.c (lambda_capture_field_type): Avoid doing auto deduction if
	the explicit initializer has parameter packs.

gcc/testsuite/ChangeLog:

	PR c++/94483
	* g++.dg/cpp2a/lambda-pack-init5.C: New test.