Merge pull request #9184 from ethereum/fixUsingForStorage

Fix using for with explicit reference types.

test/libsolidity/semanticTests/libraries/using_for_storage_structs.sol

@@ -0,0 +1,25 @@
+struct Struct { uint x; }
+
+library L {
+    function f(Struct storage _x) internal view returns (uint256) {
+      return _x.x;
+    }
+}
+
+contract C {
+  using L for Struct;
+
+  Struct s;
+
+  function h(Struct storage _s) internal view returns (uint) {
+    // _s is pointer
+    return _s.f();
+  }
+  function g() public returns (uint, uint) {
+    s.x = 7;
+    // s is reference
+    return (s.f(), h(s));
+  }
+}
+// ----
+// g() -> 7, 7