Introduce the #[diagnostic] attribute namespace

Co-authored-by: est31 <est31@users.noreply.github.com>

Co-authored-by: Esteban Kuber <estebank@users.noreply.github.com>

Co-authored-by: Vadim Petrochenkov <vadim.petrochenkov@gmail.com>

tests/ui/diagnostic_namespace/existing_proc_macros.rs

@@ -0,0 +1,24 @@
+#![feature(diagnostic_namespace)]
+// check-pass
+// aux-build:proc-macro-helper.rs
+
+extern crate proc_macro_helper;
+
+mod test1 {
+    use proc_macro_helper::diagnostic;
+
+    #[diagnostic]
+    struct Foo;
+
+}
+
+mod test2 {
+    mod diagnostic {
+        pub use proc_macro_helper::diagnostic as on_unimplemented;
+    }
+
+    #[diagnostic::on_unimplemented]
+    trait Foo {}
+}
+
+fn main() {}