This patch implements AI12-0220, which adds contracts to
Access_To_Subprogram types so that pre/postconditions are applied to all
indirect calls through such an access type.

Tested on x86_64-pc-linux-gnu, committed on trunk

2020-06-10  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* einfo.ads (Access_Subprogram_Wrapper): New attribute of
	Subprogram_Type entities. Denotes subprogram constructed for
	Access_To_Subprogram types that include pre- and postconditions.
	* einfo.adb: Subprogram bodies for Access_Subprogram_Wrapper.
	* exp_ch6.adb (Expand_Call): An indirect call through an
	Access_To_subprogram that includes contracts is rewritten as a
	call to the corresponding Access_ ubprogram_Wrapper. Handle
	derived types that inherit contract from parent.
	* sem_prag.adb (Build_Access_Subprogram_Wrapper): Build
	subprogram declaration for subprogram that incorporates the
	contracts of an Access_To_Subprogram type declaration. Build
	corresponding body and attach it to freeze actions for type.
	* sem_util.ads, sem_util.adb (Is_Access_Subprogram_Wrapper):
	Utility that uses signature of the subprogram to determine
	whether it is a generated wrapper for an Access_To_Subprogram
	type.