convert type-check constraints into NLL constraints on the fly

We used to accumulate a vector of type-check constraints, but now we
generate them as we go, and just store the NLL-style
`OutlivesConstraint` and `TypeTest` information.

src/librustc_data_structures/indexed_vec.rs

@@ -367,6 +367,11 @@ impl<I: Idx, T> IndexVec<I, T> {
         IndexVec { raw: Vec::new(), _marker: PhantomData }
     }
 
+    #[inline]
+    pub fn from_raw(raw: Vec<T>) -> Self {
+        IndexVec { raw, _marker: PhantomData }
+    }
+
     #[inline]
     pub fn with_capacity(capacity: usize) -> Self {
         IndexVec { raw: Vec::with_capacity(capacity), _marker: PhantomData }