revert changes introduced in #732 (#773)

Reverts #732. `#![feature(rustc_private)]` is necessary to execute
`c2rust-analyze`, as it utilizes `rustc` directly (see comment
[here](#772 (comment))).
This restores functionality in preparation for #772.

Author: aneksteind

analysis/tests/lighttpd-minimal/src/main.rs

@@ -1,3 +1,4 @@
+#![feature(rustc_private)]
 #![allow(non_camel_case_types)]
 #![allow(dead_code)]
 #![allow(unused_mut)]

analysis/tests/lighttpd/src/main.rs

@@ -1,3 +1,4 @@
+#![feature(rustc_private)]
 #![allow(non_upper_case_globals)]
 #![allow(non_camel_case_types)]
 #![allow(dead_code)]

analysis/tests/misc/src/main.rs

@@ -1,3 +1,4 @@
+#![feature(rustc_private)]
 #![feature(c_variadic)]
 
 #[path = "pointers.rs"]