diagnostic(…) rules are not preserved in wgsl-out

[ErichDonGubler]

Description

wgsl-in has started/will start accepting diagnostic(…) directives in #6148. Woot! Unfortunately, wgsl-out does not emit diagnostic(…) rules, even if wgsl-in input specified them. This is unfortunate, because shaders that compiled just fine in wgsl-in might not compile after a round-trip through wgsl-out.

Repro steps

1. cargo run --package naga-cli -- in.wgsl out.wgsl, where in.wgsl has the following contents:

   diagnostic(off, derivative_uniformity);

Expected vs observed behavior

We should re-emit diagnostic(…) rules in a semantics-preserving way, so users can feed wgsl-out output back into wgsl-in without introducing compilation errors.

Extra materials

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Platform

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