JavaScript: Improve XSS sanitizer detection.

We now use local data flow to detect more regexp-based sanitizers.

javascript/ql/src/semmle/javascript/security/dataflow/Xss.qll

@@ -34,7 +34,7 @@ module Shared {
     MetacharEscapeSanitizer() {
       getMethodName() = "replace" and
       exists(RegExpConstant c |
-        c.getLiteral() = getArgument(0).asExpr() and
+        c.getLiteral() = getArgument(0).getALocalSource().asExpr() and
         c.getValue().regexpMatch("['\"&<>]")
       )
     }