Extend support of _ in type parameters

- Account for `impl Trait<_>`.
 - Provide a reasonable `Span` for empty `Generics` in `impl`s.
 - Account for `fn foo<_>(_: _) {}` to suggest `fn foo<T>(_: T) {}`.
 - Fix #67995.

src/librustc_parse/parser/generics.rs

@@ -156,7 +156,7 @@ impl<'a> Parser<'a> {
             self.expect_gt()?;
             (params, span_lo.to(self.prev_span))
         } else {
-            (vec![], self.prev_span.between(self.token.span))
+            (vec![], self.prev_span.shrink_to_hi())
         };
         Ok(ast::Generics {
             params,