Rollup merge of #118123 - RalfJung:internal-lib-features, r=compiler-…

…errors

Add support for making lib features internal

We have the notion of an "internal" lang feature: a feature that is never intended to be stabilized, and using which can cause ICEs and other issues without that being considered a bug.

This extends that idea to lib features as well. It is an alternative to rust-lang/rust#115623: instead of using an attribute to declare lib features internal, we simply do this based on the name. Everything ending in `_internals` or `_internal` is considered internal.

Then we rename `core_intrinsics` to `core_intrinsics_internal`, which fixes rust-lang/rust#115597.

crates/proc-macro-srv/src/lib.rs

@@ -13,7 +13,7 @@
 #![cfg(feature = "sysroot-abi")]
 #![feature(proc_macro_internals, proc_macro_diagnostic, proc_macro_span)]
 #![warn(rust_2018_idioms, unused_lifetimes, semicolon_in_expressions_from_macros)]
-#![allow(unreachable_pub)]
+#![allow(unreachable_pub, internal_features)]
 
 extern crate proc_macro;