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

llvm-svn: 292597
diff --git a/llvm/lib/Bitcode/Reader/MetadataLoader.cpp b/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
index f76f7f8..570ff29 100644
--- a/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
+++ b/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
@@ -358,6 +358,9 @@
   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);