Rethrows checking allows invalid `try` in nested function

The following compiles, but fails at runtime with EXC_BAD_INSTRUCTION:

```swift
struct S: Error {}

func baz() throws { throw S() }

func foo(_ fn: () throws -> Void) rethrows {
  func bar() throws {
    try baz()
  }
  try bar()
}
foo({})
```

We shouldn't allow `try baz()` to count as a conditional throw for rethrowing checking.

rdar://104605214