[Ada] Spurious error -- "allocation from empty storage pool"

This patch fixes a bug in which if "pragma Default_Storage_Pool (null);"
is given, then a build-in-place function will get an incorrect error
message "allocation from empty storage pool" even though there is no
such allocation in the source program.

2018-07-31  Bob Duff  <duff@adacore.com>

gcc/ada/

	* sem_res.adb (Resolve_Allocator): Do not complain about the
	implicit allocator that occurs in the expansion of a return
	statement for a build-in-place function.

From-SVN: r263088

gcc/ada/ChangeLog

@@ -1,3 +1,9 @@
+2018-07-31  Bob Duff  <duff@adacore.com>
+
+	* sem_res.adb (Resolve_Allocator): Do not complain about the
+	implicit allocator that occurs in the expansion of a return
+	statement for a build-in-place function.
+
 2018-07-20  Martin Sebor  <msebor@redhat.com>
 
 	PR middle-end/82063

gcc/ada/sem_res.adb

@@ -5035,9 +5035,14 @@ package body Sem_Res is
          end;
       end if;
 
-      --  Check for allocation from an empty storage pool
+      --  Check for allocation from an empty storage pool. But do not complain
+      --  if it's a return statement for a build-in-place function, because the
+      --  allocator is there just in case the caller uses an allocator. If the
+      --  caller does use an allocator, it will be caught at the call site.
 
-      if No_Pool_Assigned (Typ) then
+      if No_Pool_Assigned (Typ)
+        and then not Alloc_For_BIP_Return (N)
+      then
          Error_Msg_N ("allocation from empty storage pool!", N);
 
       --  If the context is an unchecked conversion, as may happen within an