Fix std unique pointer pretty-printer for new stl versions

Summary: The unique pointer layout was changed in libstdc++ 6.0.23.

Reviewers: labath, clayborg

Reviewed By: labath, clayborg

Subscribers: luporl, lbianc, lldb-commits

Differential Revision: https://reviews.llvm.org/D44015
Patch by Alexandre Yukio Yamashita <alexandre.yamashita@eldorado.org.br>.

llvm-svn: 327017