Rollup merge of #67055 - lqd:const_qualif, r=oli-obk

Make const-qualification look at more `const fn`s

As explained in a lot more detail in #67053 this makes const-qualification not ignore the unstable const fns in libcore.

r? @oli-obk cc @ecstatic-morse

(Still a bit unsure about the `cfg`s here, for bootstrapping, does that seem correct ?)

Fixes #67053.

src/libcore/lib.rs

@@ -74,6 +74,8 @@
 #![feature(const_fn)]
 #![feature(const_fn_union)]
 #![feature(const_generics)]
+#![cfg_attr(not(bootstrap), feature(const_ptr_offset_from))]
+#![cfg_attr(not(bootstrap), feature(const_type_name))]
 #![feature(custom_inner_attributes)]
 #![feature(decl_macro)]
 #![feature(doc_cfg)]