[SILGen] Mark executor precondition check instructions as autogenerated

We don't want the debugger stepping into these.

Author: DougGregor

lib/SILGen/SILGenProlog.cpp

@@ -660,6 +660,9 @@ void SILGenFunction::emitPreconditionCheckExpectedExecutor(
   if (!checkExecutor)
     return;
 
+  // We don't want the debugger to step into these.
+  loc.markAutoGenerated();
+
   // Get the executor.
   SILValue executor = B.createExtractExecutor(loc, executorOrActor);