[naga] Write only the current entrypoint

[andyleiserson]

Changes the MSL and HLSL backends to support writing only a single entry point, and uses them that way in wgpu-hal. (The GLSL and SPIR-V backends already provide an entry point selector.)

This is working towards a fix for #5885, but this change doesn't alter override processing or validation, so we will still reject shaders for missing overrides whether or not they are used.

Testing
Both the write-one-entrypoint and write-all-entrypoints cases are exercised either by existing or changed code. Existing tests cover the change in behavior, in that the tests still work, even when no longer writing all entrypoints to the output. Additional partial-output test cases will be added with the complete fix for #5885.

Squash or Rebase? Squash

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.
• Added a changelog entry.

[cwfitzgerald]

+1 this for a different reason - it's less code to hand to the driver (for it to just throw out) and less for people looking in shader debuggers to wade through.

[teoxoy]

Entry points are uniquely identified by (name, stage), that's what the SPIR-V & GLSL backends use as well, I think the MSL & HLSL backends should follow that model to ensure only a single entry point is written.