[Ada] Enable Put_Image in pre-Ada-2020 modes

[Ada] Enable Put_Image in pre-Ada-2020 modes

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

gcc/ada/

	* exp_attr.adb (Put_Image): Remove assertion. This assertion is
	False in mixed-Ada-version programs.
	* exp_put_image.adb (Tagged_Put_Image_Enabled): New flag to make
	it easy to experiment with Put_Image on tagged types. False in
	this version.
	(Enable_Put_Image): Enable in pre-2020.  Workarounds: Disable
	for tagged types if Tagged_Put_Image_Enabled is False. Disable
	for access-to-subprogram types.  Disable if errors have been
	detected, or Sink is unavailable.
	(Preload_Sink): Move all conditionals here, from Sem_Ch10, so
	they can be nearby related code in Enable_Put_Image.  Load Sink
	only if we have seen a tagged type.  This removes the dilemma
	about calling Preload_Sink when compiling the compiler, which
	caused unwanted dependences.
	* exp_put_image.ads (Preload_Sink): New formal Compilation_Unit,
	needed to move all conditionals here, from Sem_Ch10.
	* libgnat/a-stouut.adb (Put_UTF_8): Make this suitable for
	inlining, so we don't get warnings about inlining in some tests.
	And so it can be inlined!
	* opt.ads (Tagged_Seen): New flag (see Preload_Sink).
	* scng.adb (Scan): Set new Tagged_Seen flag.
	* sem_ch10.adb (Analyze_Compilation_Unit): Move conditionals and
	comments regarding Preload_Sink into Preload_Sink.