[Ada] Compiler loop on illegal nested accept statement

This patch fixes a "Compilation abandoned" message in a compiler built
with assertions, or a compiler loop otherwise, when an accept statement
contains an illegal accept statement for the same entry.

Compiling accept_in_accept.adb must yield:

accept_in_accept.adb:12:13:
          duplicate accept statement for same entry (RM 9.5.2 (15))

----
procedure accept_in_accept is

   task a_in_a is
      entry a (i : Integer);
   end a_in_a;

   task body a_in_a is
   begin
      select
         accept a (i : Integer) do
            null;
            accept a (i : integer) do
              null;
            end a;
         end a;
      or
         terminate;
      end select;
   end a_in_a;

begin
   a_in_a.a (1);
end accept_in_accept;

2019-07-05  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_ch9.adb (Analyze_Accept_Statement): If this is an illegal
	accept statement for an enclosing entry abandon analysis to
	prevent scope mismatches and potential infinite loops in
	compiler.

From-SVN: r273129

gcc/ada/ChangeLog

+2019-07-05  Ed Schonberg  <schonberg@adacore.com>
+
+	* sem_ch9.adb (Analyze_Accept_Statement): If this is an illegal
+	accept statement for an enclosing entry abandon analysis to
+	prevent scope mismatches and potential infinite loops in
+	compiler.
+
 2019-07-05  Hristian Kirtchev  <kirtchev@adacore.com>
 
 	* ali.adb (For_Each_Invocation_Construct,

gcc/ada/sem_ch9.adb

@@ -883,7 +883,13 @@ package body Sem_Ch9 is
          exit when Task_Nam = Scope_Stack.Table (J).Entity;
 
          if Entry_Nam = Scope_Stack.Table (J).Entity then
-            Error_Msg_N ("duplicate accept statement for same entry", N);
+            Error_Msg_N
+              ("duplicate accept statement for same entry (RM 9.5.2 (15))", N);
+
+            --  Do not continue analysis of accept statement, to prevent
+            --  cascaded errors.
+
+            return;
          end if;
       end loop;