[libc++][modules] Adds the C++23 std module.

The patch is based on D144994.

D151030 added the module definitions for the module std.
This patch wires in the module and enables the basic testing.

Some notable features are missing:
- There is no test that libc++ can be fully imported as a module.
- This lacks the parts for the std.compat module.
- The module is not shipped with libc++.

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

Reviewed By: ldionne, aaronmondal, #libc

Differential Revision: https://reviews.llvm.org/D151814