Reduce verbosity of some type ascription errors

* Deduplicate type ascription LHS errors
* Remove duplicated `:` -> `::` suggestion from parse error
* Tweak wording to be more accurate
* Modify `current_type_ascription` to reduce span wrangling
* remove now unnecessary match arm
* Add run-rustfix to appropriate tests

src/librustc_parse/parser/diagnostics.rs

@@ -333,6 +333,7 @@ impl<'a> Parser<'a> {
                         Applicability::MachineApplicable
                     },
                 );
+                self.sess.type_ascription_path_suggestions.borrow_mut().insert(sp);
             } else if op_pos.line != next_pos.line && maybe_expected_semicolon {
                 err.span_suggestion(
                     sp,