[Concurrency] Look for explicit 'nonisolated' when getting isolation from protocol conformances.

[simanerush]

If the protocol from the conformance list explicitly strips off global actor with nonisolated, make sure to look for this explicit isolation source when computing the resulting isolation; to prevent accidental global actor inference on the conforming type.

Resolves rdar://144798727.

[simanerush]

@swift-ci please smoke test

[simanerush]

@swift-ci please smoke test macOS

[simanerush]

@swift-ci please smoke test

[AnthonyLatsis]

@swift-ci please smoke test macOS

[simanerush]

@swift-ci please smoke test

[xedin]

@swift-ci please test

[hborla]

I might be missing it but I think the change is missing the motivating test case, which is something like this:

struct TestIsolated: Refined {
  func test() {
  }
}

nonisolated func callTest(t: TestIsolated) {
  t.test() // okay
}

Without this change, the explicit nonisolated on the Refined protocol would not block global actor inference on TestIsolated, inferred from GloballyIsolated (which Refined refines)

[xedin]

@hborla I think that's covered in https://github.com/swiftlang/swift/pull/79893/files#diff-b3821b096ff745e478a46286c05681e14b2ede3f7b05b6a882b78b84fa03c170R148, isn't it?

[hborla]

Is that missing a protocol requirement? NonisolatedStruct does not conform to any protocols.

[xedin]

A does conform to Refined, NonisolatedStruct just uses for nonisolated function. We can add explicitly nonisolated global as well which gets A as parameter if you'd prefer.

[hborla]

A does conform to Refined, NonisolatedStruct just uses for nonisolated function. We can add explicitly nonisolated global as well which gets A as parameter if you'd prefer.

Oh I see it, I overlooked A because it isn't a new type in this PR. This is fine, thank you for explaining!

[xedin]

@swift-ci please test