Force compiler_builtins to be a private dependency

`compiler_builtins` is an injected dependency, introduced in AST as
`extern crate compiler_builtins as _`. This means that `is_private_dep`
has no way of knowing this should be private.

To get around this, always mark `compiler_builtins` private. The items
in this crate are never used

Author: tgross35

compiler/rustc_builtin_macros/src/standard_library_imports.rs

@@ -53,6 +53,8 @@ pub fn inject(
             //
             // FIXME(#113634) We should inject this during post-processing like
             // we do for the panic runtime, profiler runtime, etc.
+            //
+            // See also `is_private_dep` within `rustc_metadata`.
             cx.item(
                 span,
                 Ident::new(kw::Underscore, ident_span),

compiler/rustc_metadata/src/creader.rs

@@ -414,6 +414,12 @@ impl<'a, 'tcx> CrateLoader<'a, 'tcx> {
 
         let extern_private = self.sess.opts.externs.get(name.as_str()).map(|e| e.is_private_dep);
 
+        if name == sym::compiler_builtins {
+            // compiler_builtins is a private implementation detail and should never show up in
+            // diagnostics. See also the note referencing #113634 in
+            // `rustc_builtin_macros::...::inject`.
+            return true;
+        }
         // Any descendants of `std` should be private. These crates are usually not marked
         // private in metadata, so we ignore that field.
         if extern_private.is_none()