Specify an explicit offset when reporting an error for a for-of loop

whose target is an expression that begins with 'let'.

js/src/frontend/Parser.cpp

@@ -5536,7 +5536,7 @@ Parser<ParseHandler>::forHeadStart(YieldHandling yieldHandling,
     //
     // See ES6 13.7.
     if (isForOf && letIsIdentifier) {
-        reportWithNode(ParseError, false, *forInitialPart, JSMSG_LET_STARTING_FOROF_LHS);
+        errorAt(exprOffset, JSMSG_LET_STARTING_FOROF_LHS);
         return false;
     }