Support override-expression for workgroup_size

[kvark]

Required for https://gpuharbor.ucsc.edu/

Shader '' parsing error: expected signed integer literal of 32-bit width, found 'workgroupXSize'
    ┌─ wgsl:111:34
    │
111 │ [[stage(compute), workgroup_size(workgroupXSize)]] fn main(
    │                                  ^^^^^^^^^^^^^^ expected signed integer literal of 32-bit width

, caused by: expected signed integer literal of 32-bit width, found 'workgroupXSize'

[glalonde]

Not sure if this is the exact same issue but it seems module scope constants are explicitly allowed for workgroup_size, but naga only accepts literals.

Would be nice to do something like this:

let WORKGROUP_SIZE: u32 = 256u;
[[stage(compute), workgroup_size(WORKGROUP_SIZE)]]
fn main([[builtin(global_invocation_id)]] global_id: vec3<u32>, [[builtin(num_workgroups)]] num_workgroups: vec3<u32>) {
    let total_particles = num_workgroups[0] * num_workgroups[1] * num_workgroups[2] * WORKGROUP_SIZE;
...

Would it work to add in a branch that uses parse_const_expression in addition to parse_generic_non_negative_int_literal? or is that more complicated than it seems

[kvark]

Yeah, this can be considered a part of the issue. Technically speaking, there should be no place in code where only literals are supported. If literals are supported, then constants as well.
This part of WGSL has been evolving recently, so we couldn't know about this when just implementing the code originally. We need to update the code now.