[libc++] [test] Add a debug-mode CI.

[libc++] [test] Add a debug-mode CI.

To run llvm-lit manually from the command line:

    ./bin/llvm-lit -sv --param std=c++2b --param cxx_under_test=`pwd`/bin/clang \
        --param debug_level=1 ../libcxx/test/

Tests that currently fail with `debug_level=1` are marked `LIBCXX-DEBUG-FIXME`,
but my intent is to deal with all of them and leave no such annotations in
the codebase within the next couple weeks. (I have patches for all of them
in my local checkout.)

Differential Revision: https://reviews.llvm.org/D100866