[Ada] Accept raise expressions as operands of boolean operators

[Ada] Accept raise expressions as operands of boolean operators

This patch restores the previous behaviour of a recently rewritten
routine Sem_Ch4.Find_Boolean_Types for boolean operators where one of
the operands is a raise-expression, e.g.:

  (raise Program_Error or else (X /= Y))

This change is required for the Entity field of the "or else" operator
to be set. For GNAT it doesn't matter, but GNATprove requires this field
to be set.

The affected call sequence starts at Find_Boolean_Types, goes via
Check_Right_Argument and Check_Boolean_Pair to Valid_Boolean_Arg, which
ultimately guards a call to Add_One_Interp.

gcc/ada/

	* sem_type.adb (Valid_Boolean_Arg): Operands of Raise_Type are
	valid boolean arguments.