Fix -dynamic-too on Windows

If we're building the dynamic way too, then hasCafRefs needs to check
whether the dynamic way would use a dynamic name.

compiler/main/TidyPgm.lhs

@@ -1180,7 +1180,20 @@ hasCafRefs dflags this_pkg this_mod p arity expr
   | otherwise               = NoCafRefs
  where
   mentions_cafs = isFastTrue (cafRefsE dflags p expr)
-  is_dynamic_name = isDllName dflags this_pkg this_mod
+  is_dynamic_name n = if gopt Opt_BuildDynamicToo dflags
+                      then -- If we're building the dynamic way too,
+                           -- then we need to check whether it's a
+                           -- dynamic name there too. Note that this
+                           -- means that the vanila code is more
+                           -- pessimistic on Windows when -dynamic-too
+                           -- is used, but the alternative is that
+                           -- -dynamic-too is unusable on Windows
+                           -- as even the interfaces in the integer
+                           -- package don't match.
+                           is_dynamic_name' dflags n ||
+                           is_dynamic_name' (doDynamicToo dflags) n
+                      else is_dynamic_name' dflags n
+  is_dynamic_name' dflags' = isDllName dflags' this_pkg this_mod
   is_caf = not (arity > 0 || rhsIsStatic (targetPlatform dflags) is_dynamic_name expr)
 
   -- NB. we pass in the arity of the expression, which is expected