Like the last commit, SourceFile is used a lot by Parse and Sema, but
less so by the ClangImporter and (de)Serialization. Split it out to
cut down on recompilation times when something changes.
This commit does /not/ split the implementation of SourceFile out of
Module.cpp, which is where most of it lives. That might also be a
reasonable change, but the reason I was reluctant to is because a
number of SourceFile members correspond to the entry points in
ModuleDecl. Someone else can pick this up later if they decide it's a
good idea.
No functionality change.
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.
