Unwrap the results of type folders

Co-authored-by: Alan Egerton <eggyal@gmail.com>

compiler/rustc_middle/src/lib.rs

@@ -56,6 +56,7 @@
 #![feature(try_blocks)]
 #![feature(try_reserve_kind)]
 #![feature(nonzero_ops)]
+#![feature(unwrap_infallible)]
 #![recursion_limit = "512"]
 
 #[macro_use]