Rollup merge of #136610 - Jarcho:range_idx, r=Noratrieb

Allow `IndexSlice` to be indexed by ranges.

This comes with some annoyances as the index type can no longer inferred from indexing expressions. The biggest offender for this is `IndexVec::from_fn_n(|idx| ..., n)` where the index type won't be inferred from the call site or any index expressions inside the closure.

My main use case for this is mapping a `Place` to `Range<Idx>` for value tracking where the range represents all the values the place contains.

compiler/rustc_infer/src/infer/lexical_region_resolve/mod.rs

@@ -160,7 +160,7 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
     /// empty region. The `expansion` phase will grow this larger.
     fn construct_var_data(&self) -> LexicalRegionResolutions<'tcx> {
         LexicalRegionResolutions {
-            values: IndexVec::from_fn_n(
+            values: IndexVec::<RegionVid, _>::from_fn_n(
                 |vid| {
                     let vid_universe = self.var_infos[vid].universe;
                     VarValue::Empty(vid_universe)