[libc++][modules] Adds std module cppm files.

This adds the cppm files of D144994. These files by themselves will do
nothing. The goal is to reduce the size of D144994 and making it easier
to review the real changes of the patch.

Implements parts of
- P2465R3 Standard Library Modules std and std.compat

Reviewed By: ldionne, ChuanqiXu, aaronmondal, #libc

Differential Revision: https://reviews.llvm.org/D151030