c++: Fix VLA in template [PR95232]

c++: Fix VLA in template [PR95232]

In a non-template, grokdeclarator notices when we build a pointer to a
variably-modified type, and inserts a TYPE_DECL to make sure that type gets
gimplified.  But in a template we can't always recognize a variably-modified
type, so we need to deal with it at instantiation time.

gcc/cp/ChangeLog:

	PR c++/95232
	* cp-tree.h (predeclare_vla): Declare.
	* cp-gimplify.c (predeclare_vla): Handle getting a decl.
	* pt.c (tsubst_expr) [DECL_EXPR]: Use it.

gcc/testsuite/ChangeLog:

	PR c++/95232
	* g++.dg/ubsan/vla-2.C: New test.