Fix drop handling for if let expressions

MIR lowering for `if let` expressions is now more complicated now that
`if let` exists in HIR. This PR adds a scope for the variables bound in
an `if let` expression and then uses an approach similar to how we
handle loops to ensure that we reliably drop the correct variables.

compiler/rustc_hir/src/hir.rs

@@ -1187,6 +1187,7 @@ pub struct Arm<'hir> {
 #[derive(Debug, HashStable_Generic)]
 pub enum Guard<'hir> {
     If(&'hir Expr<'hir>),
+    // FIXME use ExprKind::Let for this.
     IfLet(&'hir Pat<'hir>, &'hir Expr<'hir>),
 }