Validate immutable variables

test/libsolidity/syntaxTests/immutable/inheritance_virtual_modifiers.sol

@@ -0,0 +1,21 @@
+contract B {
+    uint immutable x;
+
+    constructor() readX public {
+        x = 3;
+    }
+
+    modifier readX() virtual {
+        _; f(3);
+    }
+
+    function f(uint a) internal pure {}
+}
+
+contract C is B {
+    modifier readX() override {
+        _; f(x);
+    }
+}
+// ----
+// TypeError: (252-253): 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.