Introduce "SourceFile" within a TranslationUnit.

Right now this is just an extra layer of indirection for the decls,
operators, and imports in a TU, but it's the first step towards compiling
multiple source files at once without pretending they're all in a single
file. This is important for the "implicit visibility" feature, where
declarations from other source files in the same module are accessible
from the file currently being compiled.

Swift SVN r9072

lib/Sema/SourceLoader.cpp

@@ -121,7 +121,7 @@ Module *SourceLoader::loadModule(SourceLoc importLoc,
   auto *importTU = new (Ctx) TranslationUnit(moduleID.first, comp, Ctx,
                                              TranslationUnit::Library);
   Ctx.LoadedModules[moduleID.first.str()] = importTU;
-  importTU->setImportBufferID(bufferID);
+  importTU->MainSourceFile->setImportBufferID(bufferID);
 
   performAutoImport(importTU);