If a value decl is internal hasTestableOrPrivateImport will succeed (or
fail) without looking at the filename. However this breaks when we query
an internal storage decl with private formal access for a private
setter: the query would fail because no filename was serialized for the
decl (we only serialize filenames for private decls). So in the special
case of a internal storage with private accessor also serialize the
filename.
rdar://48516545
A module compiled with `-enable-private-imports` allows other modules to
import private declarations if the importing source file uses an
``@_private(from: "SourceFile.swift") import statement.
rdar://29318654
