ada: Fix handling of SPARK_Mode on standalone child subprogram

ada: Fix handling of SPARK_Mode on standalone child subprogram

SPARK_Mode aspect was not properly propagated to the body of
a standalone child subprogram from the generated spec for that subprogram,
leading GNATprove to not analyze this body. Now fixed.

gcc/ada/

	* aspects.adb (Find_Aspect): Take into account the case of a node
	of kind N_Defining_Program_Unit_Name.
	* sem_ch10.adb (Analyze_Compilation_Unit): Copy the SPARK aspect
	from the spec to the body. Delay semantic analysis after that
	point to ensure that SPARK_Mode is properly analyzed.