[LLVMContext] Detecting leaked instructions with metadata

When instructions with metadata are accidentally leaked, the result is a
difficult-to-find memory corruption in ~LLVMContextImpl that leads to
random crashes.

Patch by Arvīds Kokins!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@336010 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IR/LLVMContextImpl.cpp

@@ -48,6 +48,14 @@ LLVMContextImpl::~LLVMContextImpl() {
   while (!OwnedModules.empty())
     delete *OwnedModules.begin();
 
+#ifndef NDEBUG
+  // Check for metadata references from leaked Instructions.
+  for (auto &Pair : InstructionMetadata)
+    Pair.first->dump();
+  assert(InstructionMetadata.empty() &&
+         "Instructions with metadata have been leaked");
+#endif
+
   // Drop references for MDNodes.  Do this before Values get deleted to avoid
   // unnecessary RAUW when nodes are still unresolved.
   for (auto *I : DistinctMDNodes)