[libc++][format] Improve ABI stability.

During the review of D115991 @vitaut pointed out the enum shouldn't
depend on whether or not _LIBCPP_HAS_NO_INT128 is defined. The current
implementation lets the enum's ABI depend on this configuration option
without a good cause.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D116120