[Ada] Fix internal error on extended return and fixed-point result

[Ada] Fix internal error on extended return and fixed-point result

gcc/ada/

	* contracts.adb (Check_Type_Or_Object_External_Properties): Make
	sure to exclude all return objects from the SPARK legality rule
	on effectively volatile variables.
	* exp_ch6.adb (Expand_N_Extended_Return_Statement): Use the fast
	track only when the declaration of the return object can be
	dropped.