This separates out the bits of D11963 which were drive-by fixes, which aren't really relevant to the __config_site part of that change.
Can you move the libcpp-has-no-threads tests to test/libcxx/atomics instead of deleting them? (I imagine that's what you intended to do).
Also tested_elsewhere.pass.cpp shouldn't really be called that because it contains the actual test. Could you rename it to something?