c++: recognize in-class var tmpl partial spec [PR71954]

This makes us recognize member variable template partial specializations
defined directly inside the class body.  It seems we mainly just need to
call check_explicit_specialization when we see a static TEMPLATE_ID_EXPR
data member, which sets SET_DECL_TEMPLATE_SPECIALIZATION for us and which
we otherwise don't call (for the out-of-class case we call it from
grokvardecl).

We also need to make finish_member_template_decl return NULL_TREE for
such partial specializations, matching its behavior for class template
partial specializations, so that later we don't try to register it as a
separate member declaration.

	PR c++/71954

gcc/cp/ChangeLog:

	* decl.cc (grokdeclarator): Pass 'dname' instead of
	'unqualified_id' as the name when building the VAR_DECL for a
	static data member.  Call check_explicit_specialization for a
	TEMPLATE_ID_EXPR such member.
	* pt.cc (finish_member_template_decl): Return NULL_TREE
	instead of 'decl' when DECL_TEMPLATE_SPECIALIZATION is not
	set.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/var-templ84.C: New test.
	* g++.dg/cpp1y/var-templ84a.C: New test.