c++: Extend -Wredundant-move for const-qual objects [PR90428]

c++: Extend -Wredundant-move for const-qual objects [PR90428]

In this PR, Jon suggested extending the -Wredundant-move warning
to warn when the user is moving a const object as in:

  struct T { };

  T f(const T& t)
  {
    return std::move(t);
  }

where the std::move is redundant, because T does not have
a T(const T&&) constructor (which is very unlikely).  Even with
the std::move, T(T&&) would not be used because it would mean
losing the const.  Instead, T(const T&) will be called.

I had to restructure the function a bit, but it's better now.  This patch
depends on my other recent patches to maybe_warn_pessimizing_move.

	PR c++/90428

gcc/cp/ChangeLog:

	* typeck.cc (can_do_rvo_p): Rename to ...
	(can_elide_copy_prvalue_p): ... this.
	(maybe_warn_pessimizing_move): Extend the
	-Wredundant-move warning to warn about std::move on a
	const-qualified object.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/Wredundant-move1.C: Adjust dg-warning.
	* g++.dg/cpp0x/Wredundant-move9.C: Likewise.
	* g++.dg/cpp0x/Wredundant-move10.C: New test.