[red-knot] small efficiency improvements and bugfixes to use-def map building (#12373)

Adds inference tests sufficient to give full test coverage of the
`UseDefMapBuilder::merge` method.

In the process I realized that we could implement visiting of if
statements in `SemanticBuilder` with fewer `snapshot`, `restore`, and
`merge` operations, so I restructured that visit a bit.

I also found one correctness bug in the `merge` method (it failed to
extend the given snapshot with "unbound" for any missing symbols,
meaning we would just lose the fact that the symbol could be unbound in
the merged-in path), and two efficiency bugs (if one of the ranges to
merge is empty, we can just use the other one, no need for copies, and
if the ranges are overlapping -- which can occur with nested branches --
we can still just merge them with no copies), and fixed all three.

crates/ruff_index/src/slice.rs

@@ -80,6 +80,13 @@ impl<I: Idx, T> IndexSlice<I, T> {
         self.raw.iter_mut()
     }
 
+    #[inline]
+    pub fn iter_mut_enumerated(
+        &mut self,
+    ) -> impl DoubleEndedIterator<Item = (I, &mut T)> + ExactSizeIterator + '_ {
+        self.raw.iter_mut().enumerate().map(|(n, t)| (I::new(n), t))
+    }
+
     #[inline]
     pub fn last_index(&self) -> Option<I> {
         self.len().checked_sub(1).map(I::new)