[Ada] Put_Image improvements for strings

[Ada] Put_Image improvements for strings

2020-06-15  Bob Duff  <duff@adacore.com>

gcc/ada/

	* exp_attr.adb (Put_Image): Use underlying type for strings.
	Remove unchecked union processing.
	* exp_put_image.adb (Tagged_Put_Image_Enabled): Use -gnatd_z to
	enable default Put_Image for tagged types.  This allows testing
	that feature.
	(Build_String_Put_Image_Call): Set Conversion_OK flag.
	(Make_Component_List_Attributes): Remove unchecked union
	processing.
	(Enable_Put_Image): Disable for unchecked unions.  Enable for
	nonscalar types (which were mistakenly disabled in earlier
	changes).
	* debug.adb: Document -gnatd_z switch.
	* libgnat/s-putima.adb (Put_Image_String, Put_Image_Wide_String,
	Put_Image_Wide_Wide_String): Double double-quote characters.
	Forget about special handling of control characters for now --
	that's rare enough to not be a priority, and it's not clear what
	the right thing to do is anyway.
	* namet.adb: Minor: Improve debugger-friendliness.
	* sinfo.ads: Minor: Add "???" comment.