PR c++/90732 - ICE with VLA capture and generic lambda.

We were failing to handle VLA capture in tsubst_lambda_expr; initially
building a DECLTYPE_TYPE for the capture and then tsubsting it doesn't give
the special VLA handling.  So with this patch we call add_capture again for
VLAs.

gcc/cp/ChangeLog
2020-03-02  Jason Merrill  <jason@redhat.com>

	PR c++/90732 - ICE with VLA capture and generic lambda.
	* pt.c (tsubst_lambda_expr): Repeat add_capture for VLAs.