[SMTChecker] Correctly resolve current scope contract in VariableUsage.

test/libsolidity/smtCheckerTests/control_flow/virtual_function_call_inside_branch_1.sol

@@ -0,0 +1,16 @@
+pragma experimental SMTChecker;
+
+contract Context {}
+
+contract ERC20 is Context {
+    function approve() public virtual { _approve(); }
+    function _approve() internal virtual {}
+}
+
+contract __unstable__ERC20Owned is ERC20 {
+    function _approve() internal override {
+        if (true) {
+            super._approve();
+        }
+    }
+}

test/libsolidity/smtCheckerTests/control_flow/virtual_function_call_inside_branch_2.sol

@@ -0,0 +1,23 @@
+pragma experimental SMTChecker;
+contract A {
+    function f() internal virtual {
+        v();
+    }
+    function v() internal virtual {
+    }
+}
+
+contract B is A {
+    function f() internal virtual override {
+        super.f();
+    }
+}
+
+contract C is B {
+    function v() internal override {
+        if (0==1)
+            f();
+    }
+}
+// ----
+// Warning 6838: (303-307): BMC: Condition is always false.