c++: don't treat member var as var template

c++: don't treat member var as var template

While looking at a partial instantiation issue I noticed that we were
wrongly hitting the partial instantiation code when instantiating a static
data member of a class template.  I don't think this broke anything, but we
don't need to do that (small) extra work.

gcc/cp/ChangeLog:

	* pt.c (instantiate_decl): Only consider partial specializations of
	actual variable templates.