Replace tuple of infer vars for upvar_tys with single infer var

This commit allows us to decide the number of captures required after
completing capture ananysis, which is required as part of implementing
RFC-2229.

Co-authored-by: Aman Arora <me@aman-arora.com>
Co-authored-by: Jenny Wills <wills.jenniferg@gmail.com>

compiler/rustc_middle/src/ty/sty.rs

@@ -656,6 +656,14 @@ impl<'tcx> UpvarSubsts<'tcx> {
         };
         tupled_upvars_ty.expect_ty().tuple_fields()
     }
+
+    #[inline]
+    pub fn tupled_upvars_ty(self) -> Ty<'tcx> {
+        match self {
+            UpvarSubsts::Closure(substs) => substs.as_closure().tupled_upvars_ty(),
+            UpvarSubsts::Generator(substs) => substs.as_generator().tupled_upvars_ty(),
+        }
+    }
 }
 
 #[derive(Debug, Copy, Clone, PartialEq, PartialOrd, Ord, Eq, Hash, TyEncodable, TyDecodable)]