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_data_structures/src/lib.rs

@@ -37,6 +37,8 @@ extern crate cfg_if;
 #[macro_use]
 extern crate rustc_macros;
 
+pub use rustc_index::static_assert_size;
+
 #[inline(never)]
 #[cold]
 pub fn cold_path<F: FnOnce() -> R, R>(f: F) -> R {

compiler/rustc_data_structures/src/macros.rs

@@ -1,11 +1,3 @@
-/// Type size assertion. The first argument is a type and the second argument is its expected size.
-#[macro_export]
-macro_rules! static_assert_size {
-    ($ty:ty, $size:expr) => {
-        const _: [(); $size] = [(); ::std::mem::size_of::<$ty>()];
-    };
-}
-
 #[macro_export]
 macro_rules! enum_from_u32 {
     ($(#[$attr:meta])* pub enum $name:ident {