ada: Initial prototype of constructors

ada: Initial prototype of constructors

The patch implements the experimental constructors RFC. Currently a WIP.

gcc/ada/ChangeLog:

	* aspects.ads: Add support for constructors.
	* exp_aggr.adb: Likewise.
	* exp_attr.adb: Likewise.
	* exp_ch3.adb: Likewise.
	* exp_ch4.adb: Likewise.
	* exp_util.adb: Likewise.
	* gen_il-fields.ads: Likewise.
	* gen_il-gen-gen_entities.adb: Likewise.
	* gen_il-gen-gen_nodes.adb: Likewise.
	* par-ch4.adb: Likewise.
	* sem_aggr.adb: Likewise.
	* sem_attr.adb, sem_attr.ads: Likewise.
	* sem_ch13.adb: Likewise.
	* sem_ch3.adb: Likewise.
	* sem_ch5.adb: Likewise.
	* sem_ch6.adb: Likewise.
	* sem_res.adb: Likewise.
	* sem_util.adb, sem_util.ads: Likewise.
	* snames.ads-tmpl: Likewise.