Cleanup <__functional_03>

<__functional_03> provides the C++03 definitions for std::memfun and
std::function. However the interaction between <functional> and <__functional_03>
is ugly and duplicates code needlessly. This patch cleans up how the two
headers work together.

The major changes are:

- Provide placeholders, is_bind_expression and is_placeholder in <functional>
  for both C++03 and C++11.

- Provide bad_function_call, function fwd decl,
  __maybe_derive_from_unary_function and __maybe_derive_from_binary_function
  in <functional> for both C++03 and C++11.

- Move the <__functional_03> include to the bottom of <functional>. This makes
  it easier to see how <__functional_03> interacts with <functional>

- Remove a commented out implementation of bind in C++03. It's never going
  to get implemented.

- Mark almost all std::bind tests as unsupported in C++03. std::is_placeholder
  works in C++03 and C++11. std::is_bind_expression is provided in C++03 but
  always returns false.

llvm-svn: 242870