ada: Fix calling convention of foreign functions returning limited type

ada: Fix calling convention of foreign functions returning limited type

Such functions use neither Ada 2005's build-in-place mechanism nor Ada 95's
return-by-reference mechanism, but instead the common calling convention of
functions returning a nonlimited by-reference type.

gcc/ada/

	* exp_ch6.adb (Is_Build_In_Place_Function): Adjust comment.
	* sem_util.adb (Compute_Returns_By_Ref): Do not set Returns_By_Ref
	on functions with foreign convention.