[libc++][format] Use forwarding references.

This implements a not accepted LWG issue. Not doing so would require
integral types to use the handle class instead of being directly stored
in the basic_format_arg.

The previous code used `std::forward` in places where it wasn't required
by the Standard. These are now removed.

Implements:
- P2418R2 Add support for std::generator-like types to std::format
- LWG 3631 basic_format_arg(T&&) should use remove_cvref_t<T> throughout

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D127570