traits: Represent live alias arguments as bitsets

[Dnreikronos]

Split out of #160212 per review / Zulip: https://rust-lang.zulipchat.com/#narrow/channel/364551-t-types.2Ftrait-system-refactor/topic/rigid.20aliases.20in.20region.20handling/near/615557841

live_args_for_alias_from_outlives_bounds and args_known_to_outlive_alias_params now return identity arg indices (DenseBitSet) instead of EarlyBinder<GenericArg>. Callers just do args[idx].visit_with(...), so we stop laundering rigidness through binder instantiate. Also drops the old BitSet FIXME.

imo this is worth doing on its own even without the ICE fix. every time we shoved identity params through EarlyBinder we were writing down something we don't actually know, and this module is only going to grow. better to make the invariant explicit now than keep paying for it later.

No behavioral change intended. the rigid-alias ICE / extract_verify_if_eq bits stay on #160212. btw if that one should stack on this instead of staying independent, lmk and I'll rebase it asap.

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[Dnreikronos]

@lcnr hey, this is the bitset-only split from #160212 / the Zulip thread: https://rust-lang.zulipchat.com/#narrow/channel/364551-t-types.2Ftrait-system-refactor/topic/rigid.20aliases.20in.20region.20handling/near/615557841

fyi no matcher / ICE changes here. just arg indices via DenseBitSet, so callers do args[idx] instead of EarlyBinder + instantiate. also kills the BitSet FIXME.

imo this is the boring half and should be easy to land first. the interesting open question (proper matching vs underapproximating in extract_verify_if_eq) stays on #160212. lmk if anything looks off or if you want me to rebase that one on top of this asap.

[panstromek]

@bors try @rust-timer queue

I assume we want to test perf on this, this seems like it could have perf implications.

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[rust-bors]

☀️ Try build successful (CI)
Build commit: eb7fea4 (eb7fea435c224921ea27002d0fa50f308f2a2e00)
Base parent: e64c8a6 (e64c8a664d9da54fc239cd4404cbf67f0d624326)

[rust-timer]

Queued eb7fea4 with parent e64c8a6, future comparison URL.
There are currently 0 preceding artifacts in the queue.
It will probably take at least ~1.0 hours until the benchmark run finishes.