Rollup merge of #153483 - aytey:dyn_paren_impl_fn_return, r=fmease

Preserve parentheses around `Fn` trait bounds in pretty printer

The AST pretty printer was dropping parentheses around `Fn` trait bounds in `dyn`/`impl` types when additional `+` bounds were present. For example:

    dyn (FnMut(&mut T) -> &mut dyn ResourceLimiter) + Send + Sync

was pretty-printed as:

    dyn FnMut(&mut T) -> &mut dyn ResourceLimiter + Send + Sync

Without parens, `+ Send + Sync` binds to the inner `dyn ResourceLimiter` instead of the outer type, producing invalid Rust.

The parser already tracks parentheses via `PolyTraitRef.parens`, but `print_poly_trait_ref` never checked this field. This adds `popen()` and `pclose()` calls when `parens == Parens::Yes`.

Author: JonathanBrouwer