[libc++] Lets to_chars use header implementation.

[Mordante]

Summary

This removes the duplicated code from the dylib. Instead the dylib will
call the new functions in the header. Since this code is unneeded it's
removed from the unstable ABI.

Depends on D125704

[ldionne]

libcxx/include/__config

119–120

libcxx/src/ryu/d2s.cpp

529 ↗

(On Diff #429992)

To consider: If this increases our diff with "upstream" Ryu, it might be better to either #define __DIGIT_TABLE __itoa::__digits_base_10, or static constexpr auto& __DIGIT_TABLE = __itoa::__digits_base_10;.

[Mordante]

libcxx/src/ryu/d2s.cpp

529 ↗

(On Diff #429992)

As mentioned in the live review, since the code is in the dylib we can use inline constexpr instead of static constexpr

[Mordante]

Rebased and addresses review comments.