[Ada] Fix String_Literal aspect spec checking problem for scalars.

[Pierre-Marie de Rodat]

Result type compatibility checking for the specified String_Literal
function of a type was too strict, causing valid aspect specifications
to be incorrectly rejected.

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

gcc/ada/

        * sem_ch13.adb (Validate_Literal_Aspect): Call to Base_Type
        needed in order to correctly check result type of String_Literal
        function when the first named subtype differs from the base
        type (e.g.:
        type T is range 1 .. 10 with String_Literal => ... ;
        ).
diff --git a/gcc/ada/sem_ch13.adb b/gcc/ada/sem_ch13.adb
--- a/gcc/ada/sem_ch13.adb
+++ b/gcc/ada/sem_ch13.adb
@@ -16228,7 +16228,7 @@ package body Sem_Ch13 is
          Is_Match := False;
 
          if Ekind (It.Nam) = E_Function
-           and then Base_Type (Etype (It.Nam)) = Typ
+           and then Base_Type (Etype (It.Nam)) = Base_Type (Typ)
          then
             declare
                Params     : constant List_Id :=