Fix failing assertion (index != -1) when trying to re-declare a function parameter

Fix failing assertion (index != -1) when trying to re-declare a function parameter

Testcase: (covered in parserstress)

function foo(x) {
    var x = 42;
}

In variableDeclaration, the lookup for "x" with findMember will return -1, and
instead code for checking against arguments using indexOfArgument is needed.
The easiest fix is to simply use identifier(), which handles this accordingly.

Change-Id: I6a738d6196d4bff1fc987f111aebbaa83ed8f88f
Reviewed-by: Lars Knoll <lars.knoll@digia.com>