[libc++] Remove _LIBCPP_CONSTINIT (#171802)

`_LIBCPP_CONSTINIT` is only ever used inside `src/`, so we can just
replace it with `constinit` instead.

NOKEYCHECK=True
GitOrigin-RevId: 67cf9b8f827e816ada6dbbee3a4f0cdb7c1d0595