c++: Implement -Wself-move warning [PR81159]

c++: Implement -Wself-move warning [PR81159]

About 5 years ago we got a request to implement -Wself-move, which
warns about useless moves like this:

  int x;
  x = std::move (x);

This patch implements that warning.

	PR c++/81159

gcc/c-family/ChangeLog:

	* c.opt (Wself-move): New option.

gcc/cp/ChangeLog:

	* typeck.cc (maybe_warn_self_move): New.
	(cp_build_modify_expr): Call maybe_warn_self_move.

gcc/ChangeLog:

	* doc/invoke.texi: Document -Wself-move.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wself-move1.C: New test.