[libc++] Re-introduce special support for narrowing conversions to bool in variant (#73121)

This patch re-introduces special support for narrowing conversions to
bool
in std::variant, which was removed in 170810fc

 in order to make
libc++
Standards-conforming.

The special support is gated by the
`_LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT`
macro and will be supported for LLVM 18 only as a courtesy to help large
code bases migrate over to the Standard behavior.

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Co-authored-by: Bogdan Graur <bgraur@google.com>
Co-authored-by: Louis Dionne <ldionne.2@gmail.com>