Rename IndexVec::last → last_index

As I've been trying to replace a `Vec` with an `IndexVec`, having `last` exist on both but returning very different types makes the transition a bit awkward -- the errors are later, where you get things like "there's no `ty` method on `mir::Field`" rather than a more localized error like "hey, there's no `last` on `IndexVec`".

So I propose renaming `last` to `last_index` to help distinguish `Vec::last`, which returns an element, and `IndexVec::last_index`, which returns an index.

(Similarly, `Iterator::last` also returns an element, not an index.)

compiler/rustc_index/src/vec.rs

@@ -216,7 +216,7 @@ impl<I: Idx, T> IndexVec<I, T> {
     }
 
     #[inline]
-    pub fn last(&self) -> Option<I> {
+    pub fn last_index(&self) -> Option<I> {
         self.len().checked_sub(1).map(I::new)
     }