ada: Improve location of error messages in instantiations

ada: Improve location of error messages in instantiations

When flag -gnatdF is used, source code lines are displayed to point
the location of errors. The code of the instantiation was displayed
in case of errors inside generic instances, which was not precise.
Now the code inside the generic is displayed.

gcc/ada/

	* errout.adb (Error_Msg_Internal): Store span for Optr field, and
	adapt to new type of Optr.
	(Finalize. Output_JSON_Message, Remove_Warning_Messages): Adapt to
	new type of Optr.
	(Output_Messages): Use Optr instead of Sptr to display code
	snippet closer to error.
	* erroutc.adb (dmsg): Adapt to new type of Optr.
	* erroutc.ads (Error_Msg_Object): Make Optr a span like Sptr.
	* errutil.adb (Error_Msg): Likewise.