[libc++] Allow specifying custom Lit config files

Before this patch, the libc++ test suite first loads lit.site.cfg
(generated by CMake), and then lit.cfg. It's also possible to load
lit.cfg before lit.site.cfg and to point to a custom lit.site.cfg
file using '--param=libcxx_site_config'. However, in that case, lit.cfg
still relies on the site configuration filling up the 'config' object
like the default lit.site.cfg file does, which isn't flexible enough.

This commit simplifies the setup by having just a single Lit site config
file per CMake configuration, and always loading exactly that config file.
However, the config file to use can be selected when setting up CMake via
the LIBCXX_TEST_CONFIG setting. Furthermore, the site configs are entirely
standalone, which means that a new site config can be added that doesn't
need to conform what's expected by config.py.

Differential Revision: https://reviews.llvm.org/D81846

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