The lack of clarity manifested as unexpected behavior when using
getImportedModules to create the module import graph. The new behavior
makes SPI-ness and Shadowing-ness behave similarly in terms of
filtering. We also check if a filter is well-formed to avoid
accidental empty import lists.
This makes it easier to specify OptionSet arguments.
Also modify appropriate uses of ModuleDecl::ImportFilter to take
advantage of the new constructor.
Most of AST, Parse, and Sema deal with FileUnits regularly, but SIL
and IRGen certainly don't. Split FileUnit out into its own header to
cut down on recompilation times when something changes.
No functionality change.
