[Ada] Deal with enumeration types with very large size

[Pierre-Marie de Rodat]

It is permitted to put very large size clauses on enumeration types
so we need to deal with them in Get_Integer_Type.

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

2020-06-19  Eric Botcazou  <ebotca...@adacore.com>

gcc/ada/

        * exp_attr.adb (Get_Integer_Type): Return the largest supported
        unsigned integer type if need be.
--- gcc/ada/exp_attr.adb
+++ gcc/ada/exp_attr.adb
@@ -1766,7 +1766,7 @@ package body Exp_Attr is
             Int_Typ := Standard_Unsigned;
 
          else
-            raise Program_Error;
+            Int_Typ := Standard_Long_Long_Unsigned;
          end if;
 
          return Int_Typ;