c++: Implement CWG2387 - Linkage of const-qualified variable template [PR109126]

The following patch attempts to implement DR2387 by making variable
templates including their specialization TREE_PUBLIC when at file
scope and they don't have static storage class.

2024-08-07  Jakub Jelinek  <jakub@redhat.com>

	PR c++/109126
	* decl.cc (grokvardecl): Implement CWG 2387 - Linkage of
	const-qualified variable template.  Set TREE_PUBLIC on variable
	templates with const qualified types unless static is present.

	* g++.dg/DRs/dr2387.C: New test.
	* g++.dg/DRs/dr2387-aux.cc: New file.