Migrate to new SPIR-V reader flow

Tint has a new SPIR-V frontend that has been completely rewritten to make use of its new intermediate representation. We are planning to remove the old path in the not-too-distant future, so we recommend migrating over to the new path to make sure that everything works for your use cases.

The new flow can be accessed like this:
```
tint::wgsl::writer::ProgramOptions wgslOptions = {};
wgslOptions.allow_non_uniform_derivatives = disableUniformityAnalysis;

auto result = tint::SpirvToWgsl(spirv, wgslOptions);
if (result != tint::Success) {
std::cerr << result.Failure().reason << std::endl;
return;
}

return_string(result->wgsl.data(), result->wgsl.size());
```

Let us know if you hit any issues.

Locate the existing SPIR-V reader call sites and compare them with the new tint::SpirvToWgsl flow shown in the issue, including its ProgramOptions handling and failure result. Migrate the relevant use cases to the new path and verify that SPIR-V input still produces the expected WGSL or reports failures correctly.

Written by the indexing model from the issue text.