[Ada] Fix internal compilation error on circular type in SPARK code

[Pierre-Marie de Rodat]

When compiling or analyzing with GNATprove a piece of SPARK code with a
circular definition involving an incomplete declaration and an access to
it, the compilation/analysis may raise Program_Error in some cases. Now
fixed.

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

gcc/ada/

        * sem_ch3.adb (Access_Type_Declaration): Set Etype before
        checking for volatility compatibility.
diff --git a/gcc/ada/sem_ch3.adb b/gcc/ada/sem_ch3.adb
--- a/gcc/ada/sem_ch3.adb
+++ b/gcc/ada/sem_ch3.adb
@@ -1411,6 +1411,8 @@ package body Sem_Ch3 is
          Set_Is_Tagged_Type (T, False);
       end if;
 
+      Set_Etype (T, T);
+
       --  For SPARK, check that the designated type is compatible with
       --  respect to volatility with the access type.
 
@@ -1431,8 +1433,6 @@ package body Sem_Ch3 is
             Srcpos_Bearer => T);
       end if;
 
-      Set_Etype (T, T);
-
       --  If the type has appeared already in a with_type clause, it is frozen
       --  and the pointer size is already set. Else, initialize.