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()
}
}
```
This is a case where we used to produce:
<unknown>:0: warning: no calls to throwing functions occur within 'try' expression
Which is bogus, due to the try expr implicitly generated as part of the
implicit super.init call for an init that doesn't otherwise contain a super.init.
Silence this warning by ignoring implicitly generated trys, since this try gets
produced before name binding has resolved exactly which try is being invoked.
