ada: Fix propagation of SPARK_Mode for renaming-as-body

The value of SPARK_Mode associated with a renaming-as-body might
not be the correct one, when the private part of the package containing
the declaration has SPARK_Mode Off while the public part has SPARK_Mode
On. This may lead to analysis of code by GNATprove that should not be
analyzed.

gcc/ada/

	* freeze.adb (Build_Renamed_Body): Propagate SPARK_Pragma to body
	build from renaming, so that locally relevant value is taken into
	account.
	* sem_ch6.adb (Analyze_Expression_Function): Propagate
	SPARK_Pragma to body built from expression function, so that
	locally relevant value is taken into account.