parser: fix parsing of trait bound polarity and for-binders

The rustc AST allows both `for<>` binders and `?` polarity
modifiers in trait bounds, but they are parsed in a specific
order and validated for correctness:

  1. `for<>` binder is parsed first.
  2. Polarity modifiers (`?`, `!`) are parsed second.
  3. The parser validates that binders and polarity modifiers
     do not conflict:

```rust
if let Some(binder_span) = binder_span {
    match modifiers.polarity {
        BoundPolarity::Maybe(polarity_span) => {
            // Error: "for<...> binder not allowed with ? polarity"
        }
    }
}
```

This implies:

- `for<> ?Sized` → Valid syntax. Invalid semantics.
- `?for<> Sized` → Invalid syntax.

However, rust-analyzer incorrectly had special-case logic that
allowed `?for<>` as valid syntax. This fix removes that incorrect
special case, making rust-analyzer reject `?for<> Sized` as a
syntax error, matching rustc behavior.

This has caused confusion in other crates (such as syn) which
rely on these files to implement correct syntax evaluation.

crates/parser/src/grammar/generic_params.rs

@@ -182,12 +182,6 @@ fn type_bound(p: &mut Parser<'_>) -> bool {
             );
             m.complete(p, USE_BOUND_GENERIC_ARGS);
         }
-        T![?] if p.nth_at(1, T![for]) => {
-            // test question_for_type_trait_bound
-            // fn f<T>() where T: ?for<> Sized {}
-            p.bump_any();
-            types::for_type(p, false)
-        }
         _ => {
             if path_type_bound(p).is_err() {
                 m.abandon(p);
@@ -219,8 +213,13 @@ fn path_type_bound(p: &mut Parser<'_>) -> Result<(), ()> {
     // test async_trait_bound
     // fn async_foo(_: impl async Fn(&i32)) {}
     p.eat(T![async]);
+    // test question_for_type_trait_bound
+    // fn f<T>() where T: for<> ?Sized {}
     p.eat(T![?]);
 
+    // test_err invalid_question_for_type_trait_bound
+    // fn f<T>() where T: ?for<> Sized {}
+
     if paths::is_use_path_start(p) {
         types::path_type_bounds(p, false);
         // test_err type_bounds_macro_call_recovery