Introduce ChunkedBitSet and use it for some dataflow analyses.

This reduces peak memory usage significantly for some programs with very
large functions, such as:
- `keccak`, `unicode_normalization`, and `match-stress-enum`, from
  the `rustc-perf` benchmark suite;
- `http-0.2.6` from crates.io.

The new type is used in the analyses where the bitsets can get huge
(e.g. 10s of thousands of bits): `MaybeInitializedPlaces`,
`MaybeUninitializedPlaces`, and `EverInitializedPlaces`.

Some refactoring was required in `rustc_mir_dataflow`. All existing
analysis domains are either `BitSet` or a trivial wrapper around
`BitSet`, and access in a few places is done via `Borrow<BitSet>` or
`BorrowMut<BitSet>`. Now that some of these domains are `ClusterBitSet`,
that no longer works. So this commit replaces the `Borrow`/`BorrowMut`
usage with a new trait `BitSetExt` containing the needed bitset
operations. The impls just forward these to the underlying bitset type.
This required fiddling with trait bounds in a few places.

The commit also:
- Moves `static_assert_size` from `rustc_data_structures` to
  `rustc_index` so it can be used in the latter; the former now
  re-exports it so existing users are unaffected.
- Factors out some common "clear excess bits in the final word"
  functionality in `bit_set.rs`.
- Uses `fill` in a few places instead of loops.

compiler/rustc_mir_transform/src/remove_uninit_drops.rs

@@ -1,4 +1,4 @@
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::ChunkedBitSet;
 use rustc_middle::mir::{Body, Field, Rvalue, Statement, StatementKind, TerminatorKind};
 use rustc_middle::ty::subst::SubstsRef;
 use rustc_middle::ty::{self, ParamEnv, Ty, TyCtxt, VariantDef};
@@ -89,7 +89,7 @@ impl<'tcx> MirPass<'tcx> for RemoveUninitDrops {
 fn is_needs_drop_and_init<'tcx>(
     tcx: TyCtxt<'tcx>,
     param_env: ParamEnv<'tcx>,
-    maybe_inits: &BitSet<MovePathIndex>,
+    maybe_inits: &ChunkedBitSet<MovePathIndex>,
     move_data: &MoveData<'tcx>,
     ty: Ty<'tcx>,
     mpi: MovePathIndex,