Auto merge of #11884 - servo:antidiscrimination, r=nox

Replace usage of std::intrinsics::discriminant_value with `match`

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Replace usage of `std::intrinsics::discriminant_value` with per-enum generated code that uses `match` expressions. The LLVM IR shows that this optimizes well in release mode.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix part of #11815.

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- [ ] There are tests for these changes OR
- [x] These changes do not require new tests because they shouldn’t change any behavior.

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