Naga(msl): Clear named expressions after writing each function

[DJMcNab]

Checklist

• Run cargo clippy.
• Run cargo clippy --target wasm32-unknown-unknown if applicable.
• Add change to CHANGELOG.md. See simple instructions inside file.

Connections
This bug started in gfx-rs/naga#2309

Found in linebender/vello#398

Description
When writing the local variables for a shader, the existance of previous functions could cause expressions to be incorrectly treated as if they were named.

This change appears to match other backends, and fixes the case where expressions which were named in earlier functions are used in local variable declarations

Testing
I made a reproducing wgsl file:

fn func() -> f32 {
    return 1.0;
}

fn blend_mix() {
    func();
}

fn blend_compose(
) {
    var fa = 0.0;
}

This used to have the problem, but doesn't after this PR. Additionally, we will validate have validated that Vello works as expected (once someone with a Mac can check).

I'm not sure that a regression test is very useful here.

[raphlinus]

Confirm this works with Vello. Thanks for the fix!

[cwfitzgerald]

Dismissing erroneous wgpu review request

Admin

[teoxoy]

Makes sense, thanks!