Validate immutable variables

test/libsolidity/syntaxTests/immutable/inheritance_virtual_functions_super.sol

@@ -0,0 +1,19 @@
+contract B {
+    uint immutable x = 3;
+
+    function readX() internal view virtual returns(uint) {
+        return x;
+    }
+}
+
+contract C is B {
+    constructor() public {
+        super.readX();
+    }
+
+    function readX() internal view override returns(uint) {
+        return 1;
+    }
+}
+// ----
+// TypeError: (114-115): Immutable variables cannot be read during contract creation time, which means they cannot be read in the constructor or any function or modifier called from it.