[naga/wgsl] For loop initializer cannot be shadowed

[k2d222]

This naga/wgsl-in test is incorrect. WGSL spec does not allow shadowing the for initializer in the body. (because both declaration would have the same end-of-scope, the closing }).

fn forLexicalScope(a: f32) {
    for (var a = 0; a < 1; a++) {
        let a = true;  // <-- this is not allowed by WGSL spec.
    } // <-- because this is the end-of-scope of both declarations
    let test: f32 = a;
}

Upstream issue for reference: gpuweb/gpuweb#5024