Remove fold code and add Const::internal()

We are not planning to support user generated constant in the
foreseeable future, so we are removing the Fold logic for now in
favor of the Instance::resolve logic.

The Instance::resolve was however incomplete, since we weren't handling
internalizing constants yet. Thus, I added that.

I decided to keep the Const fields private in case we decide to
translate them lazily.

compiler/rustc_smir/src/lib.rs

@@ -13,6 +13,7 @@
 #![cfg_attr(not(bootstrap), doc(rust_logo))]
 #![cfg_attr(not(bootstrap), feature(rustdoc_internals))]
 #![cfg_attr(not(bootstrap), allow(internal_features))]
+#![allow(rustc::usage_of_ty_tykind)]
 
 pub mod rustc_internal;