WGSL: Assign to swizzle expression not allowed

[aleino-nv]

Currently the WGSL emitter will generate code that assigns to a swizzle expression, which does not seem to be allowed.

Affected tests:

• tests/language-feature/swizzles/matrix-swizzle-write-swizzle.slang
• tests/language-feature/swizzles/matrix-swizzle-write-array.slang

Example output:

WGPU error: Error while parsing WGSL: :9:5 error: cannot assign to value of type 'vec2<f32>'
    m_0[i32(0)].xy = vec2<f32>(1.0f, 4.0f);
    ^^^^^^^^^^^^^^


 - While validating [ShaderModuleDescriptor]
 - While calling [Device].CreateShaderModule([ShaderModuleDescriptor]).