Unexpected override-expression for workgroup_size

[KeKsBoTer]

Description
When setting the workgroup size with an override-expression naga reports an error:

Could not parse WGSL:
error: Unexpected override-expression
  ┌─ test.wgsl:3:26
  │
3 │ @compute @workgroup_size(block_width)
  │                          ^^^^^^^^^^^ see msg

Repro steps

1. Create a file with this minimal example (taken from the specs):

// Using an pipeline-overridable constant.
@id(42) override block_width = 12u;
@compute @workgroup_size(block_width)
fn shuffler() { }

2. Validate the shader with naga 0.20.0

Expected vs observed behavior
Naga validates shader without error.

[teoxoy]

Closing as duplicate of #4450.