A small fix for the aspect Yield defined in AI12-0279 for Ada 2022, to
accept aspect given for a subprogram body which acts as its own spec.

For example:

   procedure Switch with Yield => True is
   begin
      ...
   end Switch;

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* sem_ch13.adb (Analyze_Aspect_Yield): Look at the entity kind,
	not at the declaration kind.