ada: Get rid of unused node/entity fields (Set_ only)

ada: Get rid of unused node/entity fields (Set_ only)

The following fields are Set_ but not used in GNAT. We get rid of them
or document them as follows:

    - Has_Pragma_Controlled
	Remove.
    - Corresponding_Entry_Body
	Document that it is used by codepeer.
    - Is_Activation_Record
	Document that it is used by llvm.
    - Is_Discriminant_Check_Function
	Document that it is used by codepeer.
    - Original_Access_Type
	No change (already documented that it is used by codepeer).

gcc/ada/ChangeLog:

	* einfo.ads (Has_Pragma_Controlled): Remove.
	(Is_Activation_Record): Document usage by llvm.
	* gen_il-fields.ads (Opt_Field_Enum):
	Remove Has_Pragma_Controlled.
	* gen_il-gen-gen_entities.adb (Access_Kind):
	Remove Has_Pragma_Controlled.
	* sem_prag.adb:
	Remove setting of Has_Pragma_Controlled.
	* sinfo.ads (Corresponding_Entry_Body):
	Document usage by codepeer.