Merge C++03 and C++11 implementations of mem_fn and __mem_fn.

The implementation of mem_fn doesn't actually require any  C++11 support.
For some reason there were 17 overloads for mem_fn in C++03 when only one
is needed. This patch removes the extra overloads and uses the same implementation
of mem_fn in C++03 and C++11.

__mem_fn does require variadics to implement the call operator. Instead of
having two entirely different implementations of the __mem_fn struct, this patch
uses the same __mem_fn struct but provides different call operators when
variadics are not available.

The only thing left in <__functional_03> is the C++03 implementation of
std::function.

llvm-svn: 242959