Allow override-expressions in element count of fixed-size arrays

[ErichDonGubler]

Description

We don't appear to allow override-expressions within template parameters, though it is specified by the WebGPU spec.'s 6.2.9 Array Types section:

The element count expression N of a fixed-size array is subject to the following constraints:

• It must be an override-expression.

• It must evaluate to a concrete integer scalar.

• It is a pipeline-creation error if N is not greater than zero.

Repro steps

I don't have straightforward STR ATM (TODO), but this repros with WebGPU CTS' webgpu:shader,execution,zero_init:compute,zero_init:* test (see the source of the shader it submits here: https://github.com/mozilla/gpuweb-cts/blob/41f89e77b67e6b66cb017be4e00235a0a9429ca7/src/webgpu/shader/execution/zero_init.spec.ts#L413-L429). In particular, the var<workgroup> wg_mem : array<u32, ${wg_memory_limits} / 4>; line exercises this functionality, yielding an error like the following:

[ERROR wgpu_core::device::global] Device::create_shader_module error:
    Shader '' parsing error: expected '>', found '/'
      ┌─ wgsl:4:48
      │
    4 │       var<workgroup> wg_mem : array<u32, 16384 / 4>;
      │                                                ^ expected '>'

Expected vs observed behavior

Expected: Compilation Just Works™, and the shader module is usable in subsequently constructed pipelines.

Actual: We get a relatively unhelpful error diagnostic message, and the shader module is invalid.

Extra materials

None provided.

Platform

None provided.