[libc++] Implement format_error.

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

GitOrigin-RevId: 081c1db02dd2b48cf736d44862f7d7e8c2869577
18 files changed