[Serialization] Delay adding ModuleFiles until they're loaded

Rather than adding a ModuleFile to a parent module
and then removing it afterwards if it fails to
load, let's wait until we've loaded the file before
deciding to add it to the parent module. This then
allows us to get rid of `ModuleDecl::removeFile`.

In addition, push down the calls to `addFile` into
the callers of `loadAST` in preparation for
`addFile` being replaced with a one-time-only call
to a `setFiles` method.

tools/SourceKit/lib/SwiftLang/SwiftIndexing.cpp

@@ -195,6 +195,7 @@ static void indexModule(llvm::MemoryBuffer *Input,
       return;
     }
 
+    Mod->addFile(*FUnit);
     Mod->setHasResolvedImports();
   }