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Issue Description:

I'm writing a Swift framework library that internally uses some private C and Objective-C functions and classes. For this purpose I'm defining a "private" modulemap for an explicit nested clang module that exposes the private headers. However, it seems that the Swift compiler (or the Swift language support in Xcode) does not properly handle the import and re-export of this explicit nested module.

The attached example Xcode workspace demonstrates the following three problems:

1. The private `PrivateClass` is available in TestFramework/File2.swift although `TestFramework.Private` is not imported in that file. It seems that the import in File1.swift also improperly affects File2.swift.

2. Similarly, TestProject/ViewController.swift can use `PrivateClass` even though the nested module `TestFramework.Private` wasn't imported in that project at all. This seems again to be a side-effect of TestFramework/File1.swift importing the nested module.

3. If the "Install Objective-C Compatibility Header" build setting for the framework is set to Yes, the build fails with the unhelpful error message "No such module 'TestFramework.Private'".