Added a stripModifiers() function to remove shared|const|immutable storage classes in D2 (should eventually be moved to a dhelpers file rather than llvm helpers).

Replaced a few occurances of STCinvariant with STCimmutable.

gen/tollvm.cpp

@@ -54,9 +54,7 @@ unsigned DtoShouldExtend(Type* type)
 
 const LLType* DtoType(Type* t)
 {
-#if DMDV2
-    t = t->mutableOf();
-#endif
+    t = stripModifiers( t );
 
     if (t->irtype)
     {