Rollup merge of rust-lang#141130 - mejrs:use_self, r=compiler-errors

rustc_on_unimplemented cleanups

Addresses some of the fixmes from rust-lang#139091 and rust-lang#140307.

- switch from `_Self` to `Self` in library
- properly validate that arguments in the `on` filter and the format strings are actually valid

See rust-lang/rustc-dev-guide#2357 for the relevant documentation.

Author: matthiaskrgr

tests/ui/duplicated_attributes.rs

@@ -21,7 +21,7 @@ fn foo() {}
 fn bar() {}
 
 // No warning:
-#[rustc_on_unimplemented(on(_Self = "&str", label = "`a"), on(_Self = "alloc::string::String", label = "a"))]
+#[rustc_on_unimplemented(on(Self = "&str", label = "`a"), on(Self = "alloc::string::String", label = "a"))]
 trait Abc {}
 
 #[proc_macro_attr::duplicated_attr()] // Should not warn!