[Serialization] Check for anchoring VarDecl when serializing PDB

An invalid PDB may have no anchoring VarDecl when allowing errors, make
sure not to crash in that case.

Author: bnbarham

lib/Serialization/Serialization.cpp

@@ -2084,8 +2084,10 @@ void Serializer::writePatternBindingInitializer(PatternBindingDecl *binding,
   StringRef initStr;
   SmallString<128> scratch;
   auto varDecl = binding->getAnchoringVarDecl(bindingIndex);
+  assert((varDecl || allowCompilerErrors()) &&
+         "Serializing PDB without anchoring VarDecl");
   if (binding->hasInitStringRepresentation(bindingIndex) &&
-      varDecl->isInitExposedToClients()) {
+      varDecl && varDecl->isInitExposedToClients()) {
     initStr = binding->getInitStringRepresentation(bindingIndex, scratch);
   }
 

validation-test/Serialization/AllowErrors/invalid-pattern.swift

@@ -0,0 +1,7 @@
+// RUN: %empty-directory(%t)
+
+// -parse-as-library added so that the PDB isn't added to a TopLevelCodeDecl,
+// which isn't serialized at all
+// RUN: %target-swift-frontend -emit-module -o %t/errors.swiftmodule -module-name errors -experimental-allow-module-with-compiler-errors -parse-as-library %s
+
+let self = 1