Add an assertion to PlaceholderQueue destructor, ensuring it has been flushed

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@292597 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Bitcode/Reader/MetadataLoader.cpp

@@ -358,6 +358,9 @@ class PlaceholderQueue {
   std::deque<DistinctMDOperandPlaceholder> PHs;
 
 public:
+  ~PlaceholderQueue() {
+    assert(empty() && "PlaceholderQueue hasn't been flushed before being destroyed");
+  }
   bool empty() { return PHs.empty(); }
   DistinctMDOperandPlaceholder &getPlaceholderOp(unsigned ID);
   void flush(BitcodeReaderMetadataList &MetadataList);