Fix crash with -serialize (reported to cfe-dev). The issue was

essentially that we were destroying the declarations twice.

(Note that we don't use -serialize directly in the testsuite, only 
SerializeTest.)

llvm-svn: 51768

clang/Driver/ASTConsumers.cpp

@@ -879,6 +879,7 @@ public:
   
   virtual void Initialize(ASTContext &Context) {
     if (!TU) TU = new TranslationUnit(Context, lang);
+    TU->SetOwnsDecls(false);
   }
   
   virtual void HandleTopLevelDecl(Decl *D) {