When a property delegate type has a default initializer, use that to
implicitly initialize properties that use that delegate and do not have
their own initializers. For example, this would allow (e.g.), delegate
types like the DelayedImmutable and DelayedMutable examples to drop
the explicit initialization, e.g.,
```
@DelayedMutable var foo: Int
```
would be implicitly initialized via
```
$foo = DelayedMutable()
```
This is a simplistic implementation that does not yet propertly handle
definite initialization.
Fixes rdar://problem/50266039.