Verify that MDNodes belong to the same context as the Module.

[dexonsmith]

This makes sense; LGTM.

llvm/lib/IR/Verifier.cpp

 }
 
 void Verifier::visitMDNode(const MDNode &MD, AreDebugLocsAllowed AllowLocs) {
   // Only visit each node once.  Metadata can be mutually recursive, so this
   // avoids infinite recursion here, as well as being an optimization.
   if (!MDNodes.insert(&MD).second)
     return;
 
+  Assert(&MD.getContext() == &Context,
+         "MDNode context does not match Module context!", &MD);
+
   switch (MD.getMetadataID()) {
   default:
     llvm_unreachable("Invalid MDNode subclass");
   case Metadata::MDTupleKind:
     break;
 #define HANDLE_SPECIALIZED_MDNODE_LEAF(CLASS)                                  \
   case Metadata::CLASS##Kind:                                                  \
     visit##CLASS(cast<CLASS>(MD));                                             \

llvm/unittests/IR/VerifierTest.cpp

TEST(VerifierTest, DetectInvalidDebugInfo) {
     EXPECT_FALSE(verifyModule(M));
 
     // Now break it by not listing the CU at all.
     M.eraseNamedMetadata(M.getOrInsertNamedMetadata("llvm.dbg.cu"));
     EXPECT_TRUE(verifyModule(M));
   }
 }
 
+TEST(VerifierTest, MDNodeWrongContext) {
+  LLVMContext C1, C2;
+  auto *Node = MDNode::get(C1, None);
+
+  Module M("M", C2);
+  auto *NamedNode = M.getOrInsertNamedMetadata("test");
+  NamedNode->addOperand(Node);
+
+  std::string Error;
+  raw_string_ostream ErrorOS(Error);
+  EXPECT_TRUE(verifyModule(M, &ErrorOS));
+  EXPECT_TRUE(StringRef(ErrorOS.str())
+                  .startswith("MDNode context does not match Module context!"));
+}
+
 } // end anonymous namespace
 } // end namespace llvm