AsmWriter: Assert on unresolved metadata nodes

Assert that `MDNode::isResolved()`.  While in theory the `Verifier`
should catch this, it doesn't descend into all debug info, and the
`DebugInfoVerifier` doesn't call into the `Verifier`.  Besides, this
helps to catch bugs when `-disable-verify=true`.

Note that I haven't come across a place where this fails with clang
today, so no testcase.

llvm-svn: 232442

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -1205,6 +1205,8 @@ static void WriteModuleMetadata(const Module *M,
   SmallVector<uint64_t, 64> Record;
   for (const Metadata *MD : MDs) {
     if (const MDNode *N = dyn_cast<MDNode>(MD)) {
+      assert(N->isResolved() && "Expected forward references to be resolved");
+
       switch (N->getMetadataID()) {
       default:
         llvm_unreachable("Invalid MDNode subclass");