Rollup merge of #119216 - weiznich:use_diagnostic_namespace_in_stdlib, r=compiler-errors

Use diagnostic namespace in stdlib

This required a minor fix to have the diagnostics shown in third party crates when the `diagnostic_namespace` feature is not enabled. See 5d63f5d8d1 for details. I've opted for having a single PR for both changes as it's really not that much code. If it is required it should be easy to split up the change into several PR's.

r? `@compiler-errors`

library/core/src/lib.rs

@@ -217,6 +217,7 @@
 #![feature(const_trait_impl)]
 #![feature(decl_macro)]
 #![feature(deprecated_suggestion)]
+#![feature(diagnostic_namespace)]
 #![feature(doc_cfg)]
 #![feature(doc_cfg_hide)]
 #![feature(doc_notable_trait)]