Generate the test configuration even when LIBCXX_INCLUDE_TESTS=OFF.

This patch changes the CMake configuration so that it always
generates the test/lit.site.cfg file, even when testing is disabled.

This allows users to test libc++ without requiring them to have
a full LLVM checkout on their machine.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@296685 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed