[cxx-interop] Implicitly import Cxx module

[egorzhdan]

This lifts the requirement for the user to explicitly add import Cxx in Swift code that relies on protocols from the Cxx module, such as CxxSequence.

[egorzhdan]

@swift-ci please smoke test

[egorzhdan]

@swift-ci please smoke test

[stevapple]

Should we rename it to _Cxx to align with other implicitly imported "standard" libraries?

[egorzhdan]

@stevapple underscored names are generally used for something the developer wouldn't need to reference directly from Swift code. This module could be referenced directly, for example, if someone needs to provide a utility extension to one of the protocols from Cxx module:

// Util.swift
import Cxx

extension CxxSequence where RawIterator: ... {
  func doSomething() { ... }
}

Note that we don't always implicitly import Cxx, we only do that for C++ modules that are being imported into Swift, so without import Cxx the above snippet wouldn't compile.

[stevapple]

Ah, makes sense👌

[egorzhdan]

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[egorzhdan]

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[egorzhdan]

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[egorzhdan]

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[egorzhdan]

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[zoecarver]

Thanks for fixing all these tests!

[egorzhdan]

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[egorzhdan]

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[egorzhdan]

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[egorzhdan]

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[ktoso]

Hooray, makes a lot of sense +1

[egorzhdan]

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[egorzhdan]

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[egorzhdan]

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[egorzhdan]

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[egorzhdan]

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