[4.2 EARLY] [Serialization] Track whether a cross-reference came from Clang #17376
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…tlang#17333) Cross-references are identified by their containing module, with the assumption that two modules will never have the same name. However, an overlay has the same name as its underlying Clang module, which means that there can be two declarations with the same name, the same type, and the same module name. This is the underlying cause of the 'UIEdgeInsetsZero' problem, but it also affects the CloudKit overlay. By tracking a bit that just says "this came from Clang", we're able to resolve otherwise ambiguous cross-references. (Why didn't we do it this way all along? Because if a declaration moves from Clang to Swift or vice versa, that would break the cross-reference. But that's only interesting if the swiftmodule format is meant to be persistent across changing dependencies, and it looks like we're moving away from that anyway. It's also a little weird for SerializedModuleLoader to have special cases for Clang, but this isn't the first.) Note that I'm not reverting the UIEdgeInsetsZero workaround here; the end state will have that coming just from UIKit as originally described. rdar://problem/40839486 (cherry picked from commit 6b89415)
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jrose-apple
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Explanation: Declaration cross-references in swiftmodules are disambiguated by the target declaration's containing module, with the assumption that two modules will never have the same name. However, an overlay has the same name as its underlying Clang module, which means that there can be two declarations with the same name, the same type, and the same module name. This is the underlying cause of the 'UIEdgeInsetsZero' problem, but it also affects the CloudKit overlay. By tracking a bit that just says "this came from Clang", we're able to resolve otherwise ambiguous cross-references.
Scope: Affects how all declaration cross-references are resolved during deserialization of a swiftmodule.
Issue: rdar://problem/40839486
Risk: Medium-low. This should only affect cases that were previously "ambiguous"; that is, where a declaration defined in Swift conflicted with a declaration imported from Clang. But it is a change that exists for all cross-reference resolution.
Testing: Added compiler regression tests, verified that the original project now builds successfully.
Reviewed by: @DougGregor, @rjmccall