[Sema] Don't ignore implicit AST nodes in diagnoseUnhandledThrowSite

[theblixguy]

Resolves #61368

This partially reverts d461eab as it caused a regression where the 5.7 compiler hangs (but crashes on main), because diagnoseUnhandledThrowSite was ignoring implicit nodes, which was causing a throwing property wrapper init call to not get diagnosed since it's created implicitly.

We used to diagnose it on 5.6, but stopped doing it from 5.7 onwards.

[xedin]

@swift-ci please test

[theblixguy]

Hmm, Concurrency/async_sequence_syntax.swift is failing.

<unknown>:0: error: unexpected error produced: call can throw, but the error is not handled
<unknown>:0: error: unexpected note produced: call is to 'rethrows' function, but a conformance has a throwing witness
<unknown>:0: error: diagnostic produced elsewhere: call can throw, but the error is not handled
<unknown>:0: note: diagnostic produced elsewhere: call is to 'rethrows' function, but a conformance has a throwing witness

for

@available(SwiftStdlib 5.1, *)
func missingThrows<T : AsyncSequence>(_ seq: T) async {
  for try await _ in seq { } // expected-error{{error is not handled because the enclosing function is not declared 'throws'}}
}

I guess we're creating implicit calls to the iterator which throws? So with this change we end up emitting these extra diagnostics, probably why they don't have source locations attached to it.

[xedin]

This might be the reason why I added that check, it might be possible to add source locations to synthesized code and get this diagnostics attached to the expression at least?

[theblixguy]

@xedin Done! I added the source location of for to the synthesized next() call (reason I went with for is because that's where the throws diagnostic is attached to... although I don't think it really matters as long as it's somewhere on the for loop).

The diagnostics aren't great IMO because they actually refer to the synthesized calls, so we probably want them to be tailored to the for loop. It could be a good starter bug!

[xedin]

@swift-ci please test