Move libc++ pair/tuple assign test to libcxx/ test directory.

Libc++ implements the pair& operator=(pair<U, V>) assignment operator
using a single template that handles assignment from all tuple-like types.

This patch moves the test for that to the libcxx test directory since
it's non-standard. It also adds additional tests to the std/.../pair
directory to test the standard behavior this template implements.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@328758 91177308-0d34-0410-b5e6-96231b3b80d8