SIL: Better error when linker finds SILGen did not emit a conformance

I hit this assert a few times while working on making conformance
emission lazier, so let's at least print out the missing conformance
in question.

lib/SIL/Linker.cpp

@@ -217,8 +217,14 @@ void SILLinkerVisitor::visitProtocolConformance(
   // If the looked up witness table is a declaration, there is nothing we can
   // do here.
   if (WT == nullptr || WT->isDeclaration()) {
-    assert(!mustDeserialize &&
-           "unable to deserialize witness table when we must?!");
+#ifndef NDEBUG
+    if (mustDeserialize) {
+      llvm::errs() << "SILGen failed to emit required conformance:\n";
+      ref.dump(llvm::errs());
+      llvm::errs() << "\n";
+      abort();
+    }
+#endif
     return;
   }