[cxx-interop] Allow Swift to access non-public C++ members

[j-hui]

This patch introduces an a C++ class annotation, SWIFT_PRIVATE_FILEID, which will specify where Swift extensions of that class will be allowed to access its non-public members, e.g.:

class SWIFT_PRIVATE_FILEID("MyModule/MyFile.swift") Foo { ... };

The goal of this feature is to help C++ developers incrementally migrate the implementation of their C++ classes to Swift, without breaking encapsulation and indiscriminately exposing those classes' private and protected fields.

rdar://137764620

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This patch is a manual cherry-picking of the feature work from #77726, which got too large to be properly tested and reviewed. I first proposed this feature on Swift Forums: https://forums.swift.org/t/feature-proposal-accessing-non-public-c-members-from-swift/76116.

For those reviewing: I'd like feedback one aspect of this feature/patch, which is the name of the annotation. Specifically, I'm worried about it being named "Swift private" because that that could be confused with the existing swift_private attribute (which tells ClangImporter not to import a field).

Several other pieces of follow-up/concurrent work are needed (though none block this patch):

• Another patch to make non-public inherited members accessible in Swift. Those are not being imported yet. (This work was already done in [cxx-interop] Allow Swift extensions to access non-public C++ members #77726 so it should be pretty straightforward to port.)
• The Swift forums pitch (specifically, the gist it links to) it a bit outdated; I need to update it to reflect the current implementation (specifically, the behavior for weird annotations etc.).
• Update release notes/Swift documentation to talk about this feature.

[j-hui]

@swift-ci please smoke test