When we removed the allocator<void> specialization, the triviality of
std::allocator<void> changed because the primary template had a
non-trivial default constructor and the specialization didn't
(so std::allocator<void> went from trivial to non-trivial).
This commit fixes that oversight by giving a trivial constructor to
the primary template when instantiated on cv-void.
This was reported in https://llvm.org/PR50299.