[Ada] Crash on generic subprogram with aspect No_Return.

[Arnaud Charlet]

This patch fixes a compiler abort on a generic unit to which the aspect
No_Return applies.

Tested in ACATS 4.1D C651002.

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

2017-09-06  Ed Schonberg  <schonb...@adacore.com>

        * freeze.adb (Freeze_Entity): Do not generate a freeze
        node for a generic unit, even if it includes delayed aspect
        specifications. Freeze nodes for generic entities must never
        appear in the tree that reaches the back-end of the compiler.


Index: freeze.adb
===================================================================
--- freeze.adb  (revision 251765)
+++ freeze.adb  (working copy)
@@ -5489,6 +5489,13 @@
             then
                Explode_Initialization_Compound_Statement (E);
             end if;
+
+            --  Do not generate a freeze node for a generic unit.
+
+            if Is_Generic_Unit (E) then
+               Result := No_List;
+               goto Leave;
+            end if;
          end if;
 
       --  Case of a type or subtype being frozen