After committing D92214 it was noticed libc++ no longer builds with
C++17. For now reenable building with C++17. This is intended to be a
temporary measure in the future a C++20 capable compiler will be
required.
This is the first step at implementing <format>. It adds the <format> header
and implements the `format_error`. class.
Implemnts parts of:
-P0645 Text Formatting
Reviewed By: ldionne, #libc, miscco, curdeius
Differential Revision: https://reviews.llvm.org/D92214