c++: Fix cast to pointer to VLA.

c++: Fix cast to pointer to VLA.

The C front-end fixed this issue in r257620 by adding a DECL_EXPR from
grokdeclarator.  We don't have an easy way to do that in the C++ front-end,
but it works fine to create and prepend a DECL_EXPR when we are genericizing
the NOP_EXPR for the cast.

The C patch wraps the DECL_EXPR in a BIND_EXPR, but that seems unnecessary
in C++; this is just a hook to run gimplify_type_sizes, we aren't actually
declaring anything that we need to worry about scoping for.

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

	PR c++/88256
	* cp-gimplify.c (predeclare_vla): New.
	(cp_genericize_r) [NOP_EXPR]: Call it.