Add approximate suggestions for rustfix

This adds `span_approximate_suggestion()` that lets you emit a
suggestion marked as "approximate" in the JSON output. UI
users see no difference. This is for when rustc and clippy wish to
 emit suggestions which will make sense to the reader (e.g. they may
have placeholders like `<type>`) but are not source-applicable, so that
rustfix/etc can ignore these.

fixes #39254

src/librustc_errors/lib.rs

@@ -83,6 +83,12 @@ pub struct CodeSuggestion {
     pub substitutions: Vec<Substitution>,
     pub msg: String,
     pub show_code_when_inline: bool,
+    /// Whether or not the suggestion is approximate
+    ///
+    /// Sometimes we may show suggestions with placeholders,
+    /// which are useful for users but not useful for
+    /// tools like rustfix
+    pub approximate: bool,
 }
 
 #[derive(Clone, Debug, PartialEq, Hash, RustcEncodable, RustcDecodable)]