Warn about unreachable code.

test/libsolidity/syntaxTests/controlFlow/unreachableCode/do_while_continue.sol

@@ -0,0 +1,12 @@
+contract C {
+    function f() public pure {
+        do {
+            uint a = 42; a;
+            continue;
+            return; // this is unreachable
+        } while(false);
+        return; // this is still reachable
+    }
+}
+// ----
+// Warning: (119-126): Unreachable code.