[libc++][format] Adds a UTF transcoder.

This is a preparation for

  P2093R14 Formatted output

When the output of print is to the terminal it needs to use the native
API. This means transcoding UTF-8 to UTF-16 on Windows. The encoder's
interface is modeled after

 P2728 Unicode in the Library, Part 1: UTF Transcoding

But only the required part for P2093R14 is implemented.

On Windows wchar_t is 16 bits, in order to test on platforms where
wchar_t is 32 bits the transcoder has support for char16_t. It also adds
and UTF-8 to UTF-32 encoder which is useful for other tests.

Note it is possible to use <codecvt> for transcoding, but that header is
deprecated. So rather write new code that is not deprecated; the hard
part, decoding, has already been done. The <codecvt> header also
requires locale support while the new code works without including
<locale>.

Note the current transcoder implementation can be optimized since it
basically does UTF-8 -> UTF-32 -> UTF-16. The first goal is to have a
working implementation. Since it's not part of the ABI it's possible to
do the optimization later.

Depends on D149672

Reviewed By: ldionne, tahonermann, #libc

Differential Revision: https://reviews.llvm.org/D150031