This patch adds a separate `extensions` directory, since there are quite
a few extensions in libc++ that aren't necessarily libc++-specific. For
example, the tests currently in `libcxx/test/libcxx/extensions` should
also pass with libstdc++, since they originally added the extension.
This also "documents" what users are allowed to rely on and what parts
are just libc++ tests to make sure our implementation is behaving as we
expect, which may be subject to change.
This patch also formats the tests and refactors `.fail.cpp` tests to
`.verify.cpp` tests.
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