Rename IdxSet::clone_from.

The current situation is something of a mess.

- `IdxSetBuf` derefs to `IdxSet`.
- `IdxSetBuf` implements `Clone`, and therefore has a provided `clone_from`
  method, which does allocation and so is expensive.
- `IdxSet` has a `clone_from` method that is non-allocating and therefore
  cheap, but this method is not from the `Clone` trait.

As a result, if you have an `IdxSetBuf` called `b`, if you call
`b.clone_from(b2)` you'll get the expensive `IdxSetBuf` method, but if you call
`(*b).clone_from(b2)` you'll get the cheap `IdxSetBuf` method.
`liveness_of_locals()` does the former, presumably unintentionally, and
therefore does lots of unnecessary allocations.

Having a `clone_from` method that isn't from the `Clone` trait is a bad idea in
general, so this patch renames it as `overwrite`. This avoids the unnecessary
allocations in `liveness_of_locals()`, speeding up most NLL benchmarks, the
best by 1.5%. It also means that calls of the form `(*b).clone_from(b2)` can be
rewritten as `b.overwrite(b2)`.

src/librustc_data_structures/indexed_set.rs

@@ -233,7 +233,9 @@ impl<T: Idx> IdxSet<T> {
         &mut self.bits
     }
 
-    pub fn clone_from(&mut self, other: &IdxSet<T>) {
+    /// Efficiently overwrite `self` with `other`. Panics if `self` and `other`
+    /// don't have the same length.
+    pub fn overwrite(&mut self, other: &IdxSet<T>) {
         self.words_mut().clone_from_slice(other.words());
     }