Overhaul and separate nullptr_t tests to pass with C++03.

The standard requires that nullptr_t can be reinterpret_cast to an integral type
at least the size of nullptr_t. There is no way to emulate this conversion in
the C++03 nullptr_t implementation. The test for this conversion has been moved
to a new test and marked XFAIL with c++03.

This recommits what was originally r222296.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@222318 91177308-0d34-0410-b5e6-96231b3b80d8