iterator, allocator) constructor with the intention of it being
implicitly converted to the allocator type, it is possible for overload
resolution to favour the (iterator, iterator, enable_if) constructor.
Eliminate this possibility by moving the enable_if to one of the
existing arguments and removing the third argument.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@191145 91177308-0d34-0410-b5e6-96231b3b80d8