Allow access to `_slot` for local storage pointer variables.

test/libsolidity/semanticTests/inlineAssembly/slot_access.sol

@@ -0,0 +1,33 @@
+contract C {
+    struct S {
+        uint a;
+        uint b;
+    }
+
+    mapping(uint => S) public mappingAccess;
+
+    function data() internal view returns (S storage _data) {
+        // We need to assign it from somewhere, otherwise we would
+        // get an "uninitialized access" error.
+        _data = mappingAccess[20];
+
+        bytes32 slot = keccak256(abi.encode(uint(1), uint(0)));
+        assembly {
+            _data_slot := slot
+        }
+    }
+
+    function set(uint x) public {
+        data().a = x;
+    }
+
+    function get() public view returns (uint) {
+        return data().a;
+    }
+}
+// ----
+// get() -> 0
+// mappingAccess(uint256): 1 -> 0, 0
+// set(uint256): 4
+// get() -> 4
+// mappingAccess(uint256): 1 -> 4, 0