The C++03 version of function tried to default construct the allocator
in the uses allocator constructors when no allocation was performed. These
constructors would fail to compile when used with allocators that had no
default constructor.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@239708 91177308-0d34-0410-b5e6-96231b3b80d8