d: Use d_comdat_linkage on generated internal decl.

This adds weak linkage to internal TypeInfo data on top of the existing
DECL_COMDAT, which helps in the unlikely event that two of the same
TypeInfo data ends up in multiple places.

gcc/d/ChangeLog:

	* typeinfo.cc (TypeInfoVisitor::internal_reference): Call
	d_comdat_linkage on generated decl.

gcc/d/ChangeLog

+2020-03-31  Iain Buclaw  <ibuclaw@gdcproject.org>
+
+	* typeinfo.cc (TypeInfoVisitor::internal_reference): Call
+	d_comdat_linkage on generated decl.
+
 2020-03-22  Iain Buclaw  <ibuclaw@gdcproject.org>
 
 	PR d/93038

gcc/d/typeinfo.cc

@@ -358,7 +358,7 @@ class TypeInfoVisitor : public Visitor
     DECL_EXTERNAL (decl) = 0;
     TREE_PUBLIC (decl) = 1;
     DECL_VISIBILITY (decl) = VISIBILITY_INTERNAL;
-    DECL_COMDAT (decl) = 1;
+    d_comdat_linkage (decl);
     d_pushdecl (decl);
 
     return decl;