Parse diagnostic(…) attributes on fns

[ErichDonGubler]

Connections

• Depends on WGSL: Parse diagnostic(…); directives #6148.
• Partially resolves Support diagnostic filters in WGSL (i.e., diagnostic(…)) #5320.

Description

Building on #6148, we use all of the plumbing we made for tracking diagnostic rules, and add a parse path for @diagnostic(…) fn function() { … }. After #6148, this PR's parse path is what I've observed to be the next most common place for users to apply diagnostic filtering.

Testing

• Tests will be added to this PR before it comes out of draft.
• Demonstrate that the following CTS cases work:
  • webgpu:shader,validation,parse,diagnostic:valid_locations:*
    • …:type="attribute";location="function"
  • webgpu:shader,validation,parse,diagnostic:duplicate_attribute_same_location:*
  • webgpu:shader,validation,parse,diagnostic:conflicting_attribute_different_location:*
  • At least one case in webgpu:shader,validation,parse,diagnostic:diagnostic_scoping:*, specific case TBD.
• Port a meaningful subset of the above to our own Naga test suite, to prevent the most impactful regressions.

Checklist

• Run cargo fmt.
• Run cargo clippy. If applicable, add:
  • --target wasm32-unknown-unknown
  • --target wasm32-unknown-emscripten
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.