Warn about unreachable code.

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

@@ -0,0 +1,11 @@
+contract C {
+    function f() public pure {
+        while(true) {
+            continue;
+            return;
+        }
+        return; // this is unreachable as well, but currently undetected (needs to consider constant condition "true")
+    }
+}
+// ----
+// Warning: (100-107): Unreachable code.