libc++ makes the `hash<vector<bool, A>>::operator()` `constexpr` since
C++20, which is a conforming extension, but it was unintended.
This patch removes the extension, with an escape hatch macro for it, and
the escape hatch will be removed in the future. Test cases for
`constexpr` along with the assumption of hash values are moved to the
`libcxx/test/libcxx/` subdirectory.
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Co-authored-by: Louis Dionne <ldionne.2@gmail.com>