[naga] Gracefully handle span lookup with no module source

[andyleiserson]

Fixes #7165

Description
Gracefully handle looking up the source location for a span when no module source is available. This fixes a possible (although unlikely under normal usage) panic.

Testing
The original repro was via the wgpu crate, but rather than add a test at that level, I've added a test in wgsl_errors that seems like a reasonable approximation of the original.

I've also added a test for the InvalidReturnType error, because I didn't see an existing one, and fixed a stale comment.

Squash or Rebase? Squash, if applicable.

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests. Ran wgsl_errors locally, will let CI do the rest.
• If this contains user-facing changes, add a CHANGELOG.md entry. Technically user-facing, but doesn't seem worth a changelog entry.