[libc++] Add basic support for -std=c++2b.

* Add feature test macros.
* Add buildbot configuration generic-cxx2b that uses clang-tot.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D94227

GitOrigin-RevId: 95729f95d803c8e8f4ca3e6767404ec2c0492d53
58 files changed