[libc++] Update the documentation for running Lit to reflect reality

[libc++] Update the documentation for running Lit to reflect reality

Our documentation for running LIT is basically wrong, since it doesn't
mention `llvm-lit`, and nothing works without it. Note that this
documentation improvement reflects the current reality outside of
the recent efforts on improving the test suite. My goal is to document
the current way of running the test suite, so that everybody agrees on
how things currently work. As the configuration system for libc++ gets
simplified, we can come back and keep this documentation up to date to
reflect those changes.

Differential Revision: https://reviews.llvm.org/D77673