This is required by the Standard and makes it possible to add full
128-bit support to format.
The patch also fixes 128-bit from_chars "support". One unit test
required a too large value, this failed on 128-bit; the fix was to add
more characters to the input.
Note only base 10 has been optimized. Other bases can be optimized.
Note the 128-bit lookup table could be made smaller. This will be done later. I
really want to get 128-bit working in to_chars and format in the upcomming
LLVM 15 release, these optimizations aren't critical.
Can you please document our support for __int128_t & its unsigned counterpart under Libc++ Extensions in libcxx/docs/UsingLibcxx.rst?