[libc++] Move __thread_id out of <__threading_support>

This makes <__threading_support> closer to handling only the bridge
between the system's implementation of threading and the rest of libc++.

Differential Revision: https://reviews.llvm.org/D154464

GitOrigin-RevId: 053d9e5832c73e18bc6bf4b09e47fdd2d0ea04b7
19 files changed