A zippered framework is a single framework that can be loaded in both
macOS and macatalyst processes. Broadly to InstallAPI, it means the same
interface can represent two separate platforms.
A dylib's symbol table does not distinguish between macOS/macCatalyst.
`InstallAPI` provides the ability for the tbd file to distinct
symbols between them.
The verifier handles this special logic by tracking all unavailable and
obsoleted APIs in this context and checking against those when
determining dylib symbols with no matching declaration.
* If there exists an available decl for either platform, do not warn.
* If there is no available decl, emit a diagnostic and print the source
location for both decls.
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