[gardening] Add a missing const to a const method.

Author: gottesmm

include/swift/SILOptimizer/Analysis/Analysis.h

@@ -104,7 +104,7 @@ class SILAnalysis : public DeleteNotificationHandler {
   void unlockInvalidation() { invalidationLock = false; }
 
   /// Return True if this analysis is locked and should not be invalidated.
-  bool isLocked() { return invalidationLock; }
+  bool isLocked() const { return invalidationLock; }
 
   /// Invalidate all information in this analysis.
   virtual void invalidate() = 0;