TBDGen: Avoid crash when value witness is missing in conformance.

lib/SIL/IR/SILSymbolVisitor.cpp

@@ -287,6 +287,9 @@ class SILSymbolVisitorImpl : public ASTVisitor<SILSymbolVisitorImpl> {
       rootConformance->forEachValueWitness([&](ValueDecl *valueReq,
                                                Witness witness) {
         auto witnessDecl = witness.getDecl();
+        if (!witnessDecl)
+          return;
+
         if (isa<AbstractFunctionDecl>(valueReq)) {
           addSymbolIfNecessary(valueReq, witnessDecl);
         } else if (auto *storage = dyn_cast<AbstractStorageDecl>(valueReq)) {

test/TBD/lazy-typecheck-bad-conformance.swift

@@ -0,0 +1,10 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -typecheck -experimental-lazy-typecheck -emit-tbd -emit-tbd-path %t/lazy.tbd %s -enable-library-evolution -parse-as-library
+
+public protocol P {
+  func req()
+}
+
+// FIXME: This malformed conformance should probably be diagnosed.
+public struct S: P {
+}