c++: Extend -Wpessimizing-move for class prvalues [PR106276]

c++: Extend -Wpessimizing-move for class prvalues [PR106276]

We already have a warning that warns about pessimizing std::move
in a return statement, when it prevents the NRVO:

  T fn()
  {
    T t;
    return std::move (t); // warning \o/
  }

However, the warning doesn't warn when what we are returning is a class
prvalue, that is, when std::move prevents the RVO:

  T fn()
  {
    T t;
    return std::move (T{}); // no warning :-(
  }

This came up recently in GCC:
<https://gcc.gnu.org/pipermail/gcc-patches/2022-July/598177.html>.

This patch fixes that.  I would like to extend the warning further, so
that it warns in more contexts, e.g.:

  T t = std::move(T());

or

  void foo (T);
  foo (std::move(T()));

	PR c++/106276

gcc/cp/ChangeLog:

	* typeck.cc (can_do_rvo_p): New.
	(maybe_warn_pessimizing_move): Warn when moving a temporary object
	in a return statement prevents copy elision.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/Wpessimizing-move7.C: New test.