[silgen] Create CleanupManager::dump and the relevant dump methods to dump the current cleanups.

This is useful to discover when a specific cleanup is being eliminated while
debugging. The implementation is compiled out when assertions are disabled.

rdar://29791263

include/swift/Basic/DiverseStack.h

@@ -116,6 +116,8 @@ public:
       return Depth != (std::size_t) -1;
     }
 
+    std::size_t getDepth() const { return Depth; }
+
     /// A helper class that wraps a stable_iterator as something that
     /// pretends to be a non-null pointer.
     ///