[AST] Separate SourceFile from FileUnit.h

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.

lib/AST/Module.cpp

@@ -34,6 +34,7 @@
 #include "swift/AST/PrettyStackTrace.h"
 #include "swift/AST/PrintOptions.h"
 #include "swift/AST/ProtocolConformance.h"
+#include "swift/AST/SourceFile.h"
 #include "swift/AST/TypeCheckRequests.h"
 #include "swift/Basic/Compiler.h"
 #include "swift/Basic/SourceManager.h"