Revert accidental change to __member_pointer_traits_imp.

A previous patch accidentally made the primary template
an incomplete type. This broke some C++03 constructs.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@367762 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed