d: Move call to set_linkage_for_decl to declare_extern_var.

d: Move call to set_linkage_for_decl to declare_extern_var.

This both prevents against it being called twice for declarations that
are defined, and fixes an issue where variables defined in the
compilation get one kind of linkage (weak), and the same variables
declared via declare_extern_var get another (extern).

gcc/d/ChangeLog:

	PR d/99914
	* decl.cc (DeclVisitor::visit (StructDeclaration *)): Don't set
	DECL_INSTANTIATED on static initializer declarations.
	(DeclVisitor::visit (ClassDeclaration *)): Likewise.
	(DeclVisitor::visit (EnumDeclaration *)): Likewise.
	(d_finish_decl): Move call to set_linkage_for_decl to...
	(declare_extern_var): ...here.