[libcxx] Constrain unique_ptr::operator=(unique_ptr<Tp, Dp>) in C++03 mode

Summary:
This patch properly constrains the converting assignment operator in C++03. It also fixes a bug where std::forward was given the wrong type.
The following two tests begin passing in C++03:

* `unique_ptr.single.asgn/move_convert.pass.cpp`
* `unique_ptr.single.asgn/move_convert13.fail.cpp`

Reviewers: mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D12173

llvm-svn: 246272