[Dependency Scanning] Refactor primary scan operations into 'ModuleDependencyScanner' class

From being a scattered collection of 'static' methods in ScanDependencies.cpp
and member methods of ASTContext. This makes 'ScanDependencies.cpp' much easier
to read, and abstracts the actual scanning logic away to a place with common
state which will make it easier to reason about in the future.

lib/FrontendTool/FrontendTool.cpp

@@ -1261,10 +1261,7 @@ static bool performScanDependencies(CompilerInstance &Instance) {
     else
       return dependencies::scanDependencies(Instance);
   } else {
-    if (Instance.getInvocation().getFrontendOptions().ImportPrescan)
-      return dependencies::batchPrescanDependencies(Instance, batchScanInput);
-    else
-      return dependencies::batchScanDependencies(Instance, batchScanInput);
+    return dependencies::batchScanDependencies(Instance, batchScanInput);
   }
 }