SILGen: Partial codegen for property behaviors with DI initialization.

If a behavior has storage that can be initialized out-of-line, generate code in SILGen that uses stores to mark_uninitialized_behavior for eventual analysis by DI.

This is incomplete, particularly, it's missing code generation of glue thunks for accessors that require reabstraction, but I wanted to make sure the progress here didn't bitrot.

lib/SILGen/SILGenMaterializeForSet.cpp

@@ -437,6 +437,9 @@ void MaterializeForSetEmitter::emit(SILGenFunction &gen) {
   SILValue address;
   SILFunction *callbackFn = nullptr;
   switch (strategy) {
+  case AccessStrategy::BehaviorStorage:
+    llvm_unreachable("materializeForSet should never engage in behavior init");
+  
   case AccessStrategy::Storage:
     address = emitUsingStorage(gen, loc, self, std::move(indicesRV));
     break;