Support for returning the result of a Guard statement

[tevelee (Laszlo Teveli)]

There's prior discourse on this exact feature. Seems like my desired syntax depends on making throw an expression that returns Never.

Unwrap or Throw - Make the Safe Choice Easier

If we go the route of Never-as-universal-subtype, then it'd be reasonable to also make throw an expression of type Never, which would let you write things like x ?? throw error, as in C++, Kotlin, and other languages where throw is an expression.