[Allocator] Constrain the Deallocate templated overloads to only apply
to types which we can compute the size of. The comparison with zero
isn't actually interesting here, it's mostly about putting sizeof into
a sfinae context.

This is particular important for Deallocate as otherwise the void*
overload can quickly become ambiguous.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206251 91177308-0d34-0410-b5e6-96231b3b80d8