swift234

[Sema][SR-681] Allow catch blocks to rethrow errors when safe

A catch block can only be entered if the do block threw an error. In a
rethrows function, if the do block throws an error only under rethrows
conditions, then the catch block can only be entered under rethrows
conditions, which means the catch block can unconditionally throw and
it's still safe.

This enables code that looks like

```swift
func foo(f: () throws -> Void) rethrows {
    do {
        try f()
    } catch is SomeError {
        throw OtherError()
    }
}
```