[Distributed] Don't crash in thunk generation when missing SR conformance

[ktoso]

Problem: We would properly diagnose the missing conformance; but the compiler would still try to emit a thunk for the errored func. Tests which only -typecheck -verify would not catch this issue because it is after type checking.

The fix: This prevents distributed thunks from being generated when the func they are based on already has errors -- otherwise we can run into missing types and null pointers as we try to generate the thunk.
a
Radar: rdar://116261701

And found a follow up we should do: rdar://117677422

[ktoso]

@swift-ci please smoke test

[ktoso]

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[ktoso]

@swift-ci please smoke test macOS

Failure was unrelated:

fatal error: module map file '/home/build-user/build/buildbot_linux/swiftpm-linux-x86_64/x86_64-unknown-linux-gnu/release/llbuildBasic.build/module.modulemap' not found ci.swift.org/job/swift-PR-macos-smoke-test/9867/console

[ktoso]

@swift-ci please smoke test macOS

[ktoso]

@swift-ci please smoke test

[ktoso]

This is looking better now, thanks @slavapestov

The only case we've regressed on now is:

extension NoSerializationRequirementYet
  where SerializationRequirement: Codable {
  // expected-error@+1{{result type 'NotCodable' of distributed instance method 'test4' does not conform to serialization requirement 'Codable'}}
  distributed func test4() -> NotCodable {
    .init()
  }
}

since the getConcreteReplacementForMemberSerializationRequirement will give a null type for this... I'm not sure how to handle this, can you advise?

[ktoso]

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[ktoso]

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[ktoso]

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[slavapestov]

The RHS of a conformance requirement is always a protocol. Call requirement.getProtocolDecl() to get it

[ktoso]

Nice, thank you!

[ktoso]

done in #69874