Auto merge of #63824 - Centril:split-feature_gate, r=oli-obk

Refactor `feature_gate.rs` into modules & cleanup

Split `src/libsyntax/feature_gate.rs` into `src/libsyntax/feature_gate/` with files:
- `accepted.rs` (accepted feature gates)
- `removed.rs` (...)
- `active.rs` (...)
- `builtin_attrs.rs` (definition of builtin attributes and their gates as well as gating `cfg` flags)
- `check.rs` (post expansion checking of feature gates)
- `mod.rs` (just reexports)

Additionally, `tidy.rs` is adjusted to respect the new scheme.

Also, `builtin_attrs.rs` sees some cleanup, organization, and DSL-ification to reduce repetition.

This is probably best read commit-by-commit I think.

r? @oli-obk

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