[libc++] Constexpr-proof some machinery in not_fn.pass.cpp. NFCI.

We don't need to use global variables here; we can store the "State"
of this machinery on the stack, so that it's constexpr-friendly.

GitOrigin-RevId: 30f589c912115b4653f596eb3fd5bf62412f8aa7
1 file changed