This move the helper types `chars_format`, `to_chars_result` and
`from_chars_result` to a separate header. The first two are needed for
D70631 the third for consistency.
The header `__charconv/ryu.h` uses these types and it can't depend on the
types in `<charconv>` in a modular build. Moving them to the ryu header
would be an odd place and doesn't work since the header is included in the
middle of `<charconv>`.
Reviewed By: #libc, ldionne, Quuxplusone
Differential Revision: https://reviews.llvm.org/D108927
Mark de Wever