[libc++] Unbreak test suite (CWG1423)

clang 9ce6dc98 drops support for
implicit conversion of nullptr_t to bool. From that commit:

    The C++ rules briefly allowed this, but the rule changed nearly 10
    years ago and we never updated our implementation to match. However,
    we've warned on this by default for a long time, and no other compiler
    accepts (even as an extension).