[Ada] Add insertion character for overlay modification warnings

This enables tools that ingest GNAT's output to properly classify these
messages.

gcc/ada/

	* freeze.adb (Warn_Overlay): Add 'o' insertion character.
	* sem_ch13.adb (Analyze_Attribute_Definition_Clause): Likewise.
	* sem_util.adb (Note_Possible_Modifications): Likewise.