c++: constrained variable template issues [PR98486]

c++: constrained variable template issues [PR98486]

This fixes some issues with constrained variable templates:

  - Constraints aren't checked when explicitly specializing a variable
    template.
  - Constraints aren't attached to a static data member template at
    parse time.
  - Constraints don't get propagated when (partially) instantiating a
    static data member template, so we need to make sure to look up
    constraints using the most general template during satisfaction.

	PR c++/98486

gcc/cp/ChangeLog:

	* constraint.cc (get_normalized_constraints_from_decl): Always
	look up constraints using the most general template.
	* decl.c (grokdeclarator): Set constraints on a static data
	member template.
	* pt.c (determine_specialization): Check constraints on a
	variable template.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-var-templ1.C: New test.
	* g++.dg/cpp2a/concepts-var-templ1a.C: New test.
	* g++.dg/cpp2a/concepts-var-templ1b.C: New test.

(cherry picked from commit 2e2e65a46d2674bed53afd211493876ee2b79453)