[libcxx] [test] D26815: Fix an assumption about the state of moved-from std::functions.

The Standard doesn't provide any guarantees beyond "valid but unspecified" for
moved-from std::functions. libcxx moves from small targets and leaves them
there, while MSVC's STL empties out the source. Mark these assertions as
libcxx-specific.

llvm-svn: 287382