[Ada] Fixes for GNAT error/warning messages

[Ada] Fixes for GNAT error/warning messages

gcc/ada/

	* checks.adb: Rework error messages.
	* exp_ch3.adb: Likewise.
	* freeze.adb: Likewise.
	* lib-load.adb: Likewise.
	* par-ch12.adb: Likewise.
	* par-ch3.adb: Likewise.
	* par-ch4.adb: Likewise.
	* par-ch9.adb: Likewise.
	* sem_aggr.adb: Likewise.
	* sem_attr.adb: Likewise.
	* sem_cat.adb: Likewise.
	* sem_ch10.adb: Likewise.
	* sem_ch12.adb: Likewise.
	(Instantiate_Type): Fix CODEFIX comment, applicable only on
	continuation message, and identify the second message as a
	continuation.
	* sem_ch13.adb: Rework error messages.
	* sem_ch3.adb: Likewise.
	* sem_ch4.adb: Likewise.
	* sem_ch5.adb: Likewise.
	* sem_ch6.adb: Likewise.
	* sem_ch8.adb: Likewise.
	* sem_ch9.adb: Likewise.
	* sem_prag.adb: Likewise.
	* sem_res.adb: Likewise.
	* sem_util.adb: Likewise.
	(Wrong_Type): Fix CODEFIX comment, applicable only on
	continuation message, and identify the second message as a
	continuation.
	* symbols.adb: Rework error messages.

gcc/testsuite/

	* gnat.dg/interface6.adb, gnat.dg/not_null.adb,
	gnat.dg/protected_func.adb: Adjust error messages.