c++: avoid non-TARGET_EXPR class prvalues

c++: avoid non-TARGET_EXPR class prvalues

Around PR98469 I asked Jakub to wrap a class BIT_CAST_EXPR in TARGET_EXPR;
SPACESHIP_EXPR needs the same thing.  The dummy CAST_EXPR created in
can_convert is another instance of a non-TARGET_EXPR prvalue, so let's use
the declval-like build_stub_object there instead.

gcc/cp/ChangeLog:

	* cp-tree.h (build_stub_object): Declare.
	* method.c (build_stub_object): No longer static.
	* call.c (can_convert): Use it.
	* tree.c (build_dummy_object): Adjust comment.
	* typeck.c (cp_build_binary_op): Wrap SPACESHIP_EXPR in a
	TARGET_EXPR.