Variadic generic default argument does not work for single element cases

This works:

```swift
func foo<each T>(xs: (repeat each T) = (0, 0)) {}
```

This doesn't:

```swift
func foo<each T>(xs: (repeat each T) = 0) {}
// error: Default argument value of type 'Int' cannot be converted to type '(repeat each T)'
```

```
Swift version 6.2-dev (LLVM 872428812ba43af, Swift 9ed8b664fd017bc)
```