add the prepareInitialization builtin.

It is like `zeroInitializer`, but does not actually initialize the memory.
It only indicates to mandatory passes that the memory is going to be initialized.

lib/SILOptimizer/Transforms/AccessEnforcementReleaseSinking.cpp

@@ -135,6 +135,7 @@ static bool isBarrier(SILInstruction *inst) {
     case BuiltinValueKind::IsNegative:
     case BuiltinValueKind::WordAtIndex:
     case BuiltinValueKind::ZeroInitializer:
+    case BuiltinValueKind::PrepareInitialization:
     case BuiltinValueKind::Once:
     case BuiltinValueKind::OnceWithContext:
     case BuiltinValueKind::GetObjCTypeEncoding: