[libc++] Unbreak test suite (CWG1423)
clang 9ce6dc9872be4081fb98f6161c28581e1cbbe7dc 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).