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_mir_transform/src/coverage/tests.rs

@@ -65,7 +65,7 @@ impl<'tcx> MockBlocks<'tcx> {
     }
 
     fn push(&mut self, kind: TerminatorKind<'tcx>) -> BasicBlock {
-        let next_lo = if let Some(last) = self.blocks.last() {
+        let next_lo = if let Some(last) = self.blocks.last_index() {
             self.blocks[last].terminator().source_info.span.hi()
         } else {
             BytePos(1)