[libc++][format] Fixes visit_format_arg.

The Standard specifies which types are stored in the basic_format_arg
"variant" and which types are stored as a handle. Libc++ stores
additional types in the "variant". During a reflector discussion
@jwakely mention this is user observable; visit_format_arg uses the type
instead of a handle as argument.

This optimization is useful and will probably be used for other small
types in the future. To be conferment the visitor creates a handle and
uses that as argument. There is a second visitor so the formatter can
still directly access the 128-bit integrals.

The test for the visitor and get has been made public too, there is no
reason not too. The 128-bit integral types are required by the Standard,
when they are available.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D138052