ImproperCTypes: misc. adaptations

smooth things out to avoid conflicts with
rust-lang/compiler-builtins#1006
which has at time of writing not made it into rust-lang/rust's main
branch

Author: niacdoial

tests/auxiliary/minicore.rs

@@ -29,7 +29,7 @@
     asm_experimental_arch,
     unboxed_closures
 )]
-#![allow(unused, improper_ctypes_definitions, internal_features)]
+#![allow(unused, internal_features)]
 #![no_std]
 #![no_core]
 

tests/ui/statics/nested-allocations-dont-inherit-codegen-attrs.rs

@@ -1,6 +1,6 @@
 //@ build-pass
 
-#![allow(improper_c_var_definitions)]
+#![allow(improper_ctypes_definitions)]
 
 // Make sure that the nested static allocation for `FOO` doesn't inherit `no_mangle`.
 #[no_mangle]