[NFC] Remove non-rvlaue non-variadic allocator::construct overloads.

Summary: All supported compilers have rvalues and variadics so we can safely remove the overloads of allocator::construct which are only enabled on compilers without rvalues and variadics.

Reviewers: ldionne, #libc!

Subscribers: dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D80068