[libc++] Add from-scratch configuration files for the test suite

This commit adds the first from-scratch configuration files for running
the libc++ test suite without using the old configuration:

- libcxx-trunk-shared.cfg.py:
   Runs the test suite against a trunk libc++ shared library.
- libcxx-trunk-static.cfg.py:
   Runs the test suite against a trunk libc++ static library.

Differential Revision: https://reviews.llvm.org/D81866