Allow Deriving Type Witnesses Across Files

[CodaFi]

A recent change to witness matching in #32578 suddenly made the
following construction illegal:

// File1.swift/SomeBridgingHeader.h
enum MyEnumInAnotherFile { /**/ }

// File2.swift
extension MyEnumInAnotherFile {
  static var allCases: [MyEnumInAnotherFile] { /**/ }
}

Because it was no longer possible to derive the type witness for
AllCases. This is because, when inference ran before synthesis, we
would use the value witness to pick out the type witness and thus had no
need for synthesis. Now that we run synthesis first, we ought to just
allow deriving type witnesses across files, but still ban deriving value
witnesses. In general, if you can utter a type in a different file to
extend it, you should be able to see the requirements necessary to
derive a default type witness.

rdar://66279278, rdar://66279375, rdar://66279384, rdar://66279415, rdar://66279503, rdar://65943338

[CodaFi]

@swift-ci smoke test

[CodaFi]

@swift-ci test source compatibility

[CodaFi]

The emission of cannot_synthesize_in_crossfile_extension now occurs once per value witness. This is unfortunate but only because of the wording of this diagnostic. We need to mention the specific value witness that we cannot synthesize here. I'll see about improving it in a follow-up commit. For now, we need to undo the source break.

[CodaFi]

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