ada: Fix Unchecked_Conversion in edge case

ada: Fix Unchecked_Conversion in edge case

Before this patch, Set_Can_Use_Internal_Rep was called on access
to subprogram subtypes when instantiating Unchecked_Conversion
from System.Address to an access to subprogram subtype (or the
reverse). This was incorrect and caused an assertion failure.

This patch fixes that by modifying the Can_Use_Internal_Rep
attribute of the base type of the subtype instead.

gcc/ada/

	* sem_ch13.adb (Validate_Unchecked_Conversion): Fix behavior on
	System.Address to access to subprogram subtype conversion.