[Ada] Cleanups related to expansion of dispatching primitives

[Ada] Cleanups related to expansion of dispatching primitives

gcc/ada/

	* doc/gnat_rm/standard_and_implementation_defined_restrictions.rst
	(No_Dispatching_Calls): Fix whitespace in example code.
	* gnat_rm.texi: Regenerate.
	* exp_ch13.adb (Expand_N_Freeze_Entity): Replace low-level
	membership test with a high-level wrapper.
	* exp_ch3.adb (Expand_Freeze_Record_Type): Remove unnecessary
	initialization of list of wrapper declarations and unnecessary
	guard for list of their bodies (if no bodies are created then
	Append_Freeze_Actions is a no-op).