[Ada] Put_Image: preload type Sink, change what types have it enabled

[Ada] Put_Image: preload type Sink, change what types have it enabled

2020-06-12  Bob Duff  <duff@adacore.com>

gcc/ada/

	* exp_put_image.ads, exp_put_image.adb (Preload_Sink): Procedure
	for preloading type Sink.  This has the side effect of loading
	various dependents, including Ada.Strings.UTF_Encoding.
	(Enable_Put_Image): Disable Put_Image in pre-2020 versions of
	Ada.  This limitation can probably be lifted later.  Enable for
	tagged types except in predefined units.  Disable for CPP types;
	Put_Image is legal, just prints the type name.
	* sem_attr.adb (Check_Put_Image_Attribute): Don't complain about
	Put_Image of CPP types; instead call the "unknown" version of
	Put_Image.
	* sem_ch10.adb (Analyze_Compilation_Unit): Call Preload_Sink.