Include the parent `ModuleDecl` when serializing a `SILFunction` so that it is available on deserialized functions even though the full `DeclContext` is not present. With the parent module always available we can reliably compute whether the `SILFunction` comes from a module that was imported `@_weakLinked`.
Serialize the `DeclContext` member of `SILFunction` so that it can be used to look up the module that a function belongs to in order to compute weak import status.
Resolves rdar://98521248
