c++: use conversion_obstack_sentinel throughout

This replaces direct calls to conversion_obstack_alloc(0) and obstack_free
with the recently added conversion_obstack_sentinel.  In passing, I
noticed build_user_type_conversion and build_operator_new_call don't
free their conversion_obstack allocations, so this patch also uses this
SFINAE helper in those two entry points.

gcc/cp/ChangeLog:

	* call.cc (build_user_type_conversion): Free allocated
	conversions.
	(build_converted_constant_expr_internal): Use
	conversion_obstack_sentinel instead.
	(perform_dguide_overload_resolution): Likewise.
	(build_new_function_call): Likewise.
	(build_operator_new_call): Free allocated conversions.
	(build_op_call): Use conversion_obstack_sentinel instead.
	(build_conditional_expr): Use conversion_obstack_sentinel
	instead, and hoist it out to the outermost scope.
	(build_new_op): Use conversion_obstack_sentinel instead
	and set it up before the first goto.  Remove second unneeded goto.
	(build_op_subscript): Use conversion_obstack_sentinel instead.
	(ref_conv_binds_to_temporary): Likewise.
	(build_new_method_call): Likewise.
	(can_convert_arg): Likewise.
	(can_convert_arg_bad): Likewise.
	(perform_implicit_conversion_flags): Likewise.
	(perform_direct_initialization_if_possible): Likewise.
	(initialize_reference): Likewise.

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