Fixing ICE when returning Struct from library

test/libsolidity/semanticTests/libraries/library_return_struct_with_mapping.sol

@@ -0,0 +1,21 @@
+pragma experimental ABIEncoderV2;
+
+library Lib {
+    struct Items {
+        mapping (uint => uint) a;
+    }
+
+    function get() public returns (Items storage x) {
+        assembly { x.slot := 123 }
+    }
+}
+
+contract C {
+    function f() public returns(uint256 slot) {
+        Lib.Items storage ptr = Lib.get();
+        assembly { slot := ptr.slot }
+    }
+}
+// ----
+// library: Lib
+// f() -> 123