c++: extend -Wself-move for mem-init-list [PR109396]

c++: extend -Wself-move for mem-init-list [PR109396]

We already warn for:

  x = std::move (x);

which triggers:

  warning: moving 'x' of type 'int' to itself [-Wself-move]

but bug 109396 reports that this doesn't work for a member-initializer-list:

  X() : x(std::move (x))

so this patch amends that.

	PR c++/109396

gcc/cp/ChangeLog:

	* cp-tree.h (maybe_warn_self_move): Declare.
	* init.cc (perform_member_init): Call maybe_warn_self_move.
	* typeck.cc (maybe_warn_self_move): No longer static.  Change the
	return type to bool.  Also warn when called from
	a member-initializer-list.  Drop the inform call.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wself-move2.C: New test.