[Serialization] Make sure we handle all decl kinds

...well, at least a little more than before. This /still/ isn't as
strong as the switch that used to be here because a subtype of an
existing Decl or Type won't show up, but that's probably still worth
the maintainability (and backtrace) tradeoff.

lib/Serialization/Serialization.cpp

@@ -2848,9 +2848,8 @@ class Serializer::DeclSerializer : public DeclVisitor<DeclSerializer> {
     DeclVisitor<DeclSerializer>::visit(const_cast<Decl *>(D));
   }
 
-  void visitDecl(const Decl *) {
-    llvm_unreachable("I guess we forgot a Decl kind?");
-  }
+  /// If this gets referenced, we forgot to handle a decl.
+  void visitDecl(const Decl *) = delete;
 
   void visitExtensionDecl(const ExtensionDecl *extension) {
     using namespace decls_block;