[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.
diff --git a/gcc/ada/exp_imgv.adb b/gcc/ada/exp_imgv.adb
--- a/gcc/ada/exp_imgv.adb
+++ b/gcc/ada/exp_imgv.adb
@@ -483,7 +483,7 @@ package body Exp_Imgv is
       --  underlying type.
 
       if Ada_Version >= Ada_2020 then
-         Rtyp := Underlying_Type (Ptyp);
+         Rtyp := Underlying_Type (Root_Type (Ptyp));
       else
          Rtyp := Root_Type (Ptyp);
       end if;