Rename optin_builtin_traits to auto_traits

They were originally called "opt-in, built-in traits" (OIBITs), but
people realized that the name was too confusing and a mouthful, and so
they were renamed to just "auto traits". The feature flag's name wasn't
updated, though, so that's what this PR does.

There are some other spots in the compiler that still refer to OIBITs,
but I don't think changing those now is worth it since they are internal
and not particularly relevant to this PR.

Also see <https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/opt-in.2C.20built-in.20traits.20(auto.20traits).20feature.20name>.

example/mini_core.rs

@@ -1,6 +1,6 @@
 #![feature(
     no_core, lang_items, intrinsics, unboxed_closures, type_ascription, extern_types,
-    untagged_unions, decl_macro, rustc_attrs, transparent_unions, optin_builtin_traits,
+    untagged_unions, decl_macro, rustc_attrs, transparent_unions, auto_traits,
     thread_local,
 )]
 #![no_core]