[Ada] Clean up error handling of 'Image

[Pierre-Marie de Rodat]

...in preparation for enabling 'Image for all types in Ada 2020, and
having it call 'Put_Image for nonscalar types, and for types with
user-defined 'Put_Image.

Tested on x86_64-pc-linux-gnu, committed on trunk

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

gcc/ada/

	* sem_attr.adb (Check_Image_Type): New procedure for checking
	the type, depending on language version. Disable the Ada 2020
	support until the corresponding expander work is done.
	(Analyze_Image_Attribute): Call Check_Image_Type.  Rearrange the
	code to be simplier and more logical.  When P_Type is modified,
	modify P_Base_Type accordingly.
	* sem_util.adb (Is_Object_Image): Do not return False if the
	prefix is a type. X'Image should be considered an image of an
	object iff X is an object (albeit illegal pre-2020 if
	nonscalar).
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