SILgenPattern: Mark location used by forwardIntoSubtree as autogenerated.

If a pattern reuses a value from a previous branch this could result in a step
backwords or a previous scope being reopeneed.

rdar://107764966
(cherry picked from commit 68aa944)

Author: adrian-prantl

lib/SILGen/SILGenPattern.cpp

@@ -741,6 +741,7 @@ static ConsumableManagedValue
 forwardIntoSubtree(SILGenFunction &SGF, SILLocation loc,
                    CleanupStateRestorationScope &scope,
                    ConsumableManagedValue outerCMV) {
+  loc.markAutoGenerated();
   ManagedValue outerMV = outerCMV.getFinalManagedValue();
   if (!outerMV.hasCleanup()) return outerCMV;