Eliminate the C++0x-only is_convertible testing function that accepts

a cv-qualifier rvalue reference to the type, e.g.,

  template <class _Tp> char  __test(const volatile typename remove_reference<_Tp>::type&&);

The use of this function signature rather than the more
straightforward one used in C++98/03 mode, e.g.,

  template <class _Tp> char  __test(_Tp);

is broken in two ways:

  1) An rvalue reference cannot bind to lvalues, so is_convertible<X&,
  X&>::value would be false. This breaks two of the unique_ptr tests
  on Clang and GCC >= 4.5. Prior GCC's seem to have allowed rvalue
  references to bind to lvalues, allowing this bug to slip in.

  2) By adding cv-qualifiers to the type we're converting to, we get
  some incorrect "true" results for, e.g., is_convertible<const X&, X&>::value.

llvm-svn: 124166