Implement full support for non-pointer types in custom allocators.  This is for the associative containers only.  This work still needs to be done on the unordered and sequence containers.  Fixes http://llvm.org/bugs/show_bug.cgi?id=15978

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@184358 91177308-0d34-0410-b5e6-96231b3b80d8