add cli arg to choose metal version

[pcleavelin]

Connections
Addresses #5389

Description
naga-cli has no way to select an MSL version.

Testing
Try to compile a .wgsl shader to a .metal shader that requires features only present in MSL versions >1.0 using the new --metal-version argument.
As an example, using the @builtin(instance_index) attribute:

struct Input {
    @location(0) position: vec4<f32>,
    @builtin(instance_index) instance_id: u32,
}

struct Output {
    @builtin(position) position: vec4<f32>,
}

@vertex
fn vs_main(input: Input) -> Output {
    var output: Output;

    output.position = input.position;

    return output;
}

and running

$ naga test.wgsl test.metal --metal-version 1.2

Checklist

• Run cargo fmt.
• Run cargo clippy. If applicable, add:
  • --target wasm32-unknown-unknown
  • --target wasm32-unknown-emscripten
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.