[Ada] Bug in Enum_Subtype'Image in Ada 2020 mode

[Pierre-Marie de Rodat]

This patch fixes a bug in which Enum_Subtype'Image, where Enum_Subtype
is a non-first subtype of an enumeration type, would return the image of
the 'Pos -- that is, an integer instead of the text of the enumeration
literal.

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

gcc/ada/

	* exp_imgv.adb (Expand_Image_Attribute): Add Root_Type, so
	constrained subtypes work.
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