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 https://github.com/rust-lang/rust/pull/119216/commits/5d63f5d8d1a72167c1d5242b2e1ed5b7259fd526 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`
d90c702566