Revert accidental change to __member_pointer_traits_imp.
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: 756ef344af921d95d562d9e9f9389127a89a6314@367762 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx