Separate dependency tracking from Make-style .d file generation.

This refactoring is groundwork for saving the cross-module dependencies
in the swiftdeps files as well, so that we know to rebuild files if an
outside file changes (such as a bridging header, another framework's
headers, or another framework's swiftmodule).

Part of rdar://problem/19270920

Swift SVN r24258

include/swift/Frontend/DependencyFileGenerator.h

@@ -1,34 +0,0 @@
-//===- DependencyFileGenerator.h - .d file generation -----------*- C++ -*-===//
-//
-// This source file is part of the Swift.org open source project
-//
-// Copyright (c) 2014 - 2015 Apple Inc. and the Swift project authors
-// Licensed under Apache License v2.0 with Runtime Library Exception
-//
-// See http://swift.org/LICENSE.txt for license information
-// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
-//
-//===----------------------------------------------------------------------===//
-
-#ifndef SWIFT_FRONTEND_DEPENDENCYFILEGENERATOR_H
-#define SWIFT_FRONTEND_DEPENDENCYFILEGENERATOR_H
-
-#include "swift/AST/ModuleLoader.h"
-#include "swift/Basic/LLVM.h"
-#include "llvm/ADT/SetVector.h"
-#include "llvm/ADT/StringSet.h"
-#include <vector>
-
-namespace swift {
-  class DependencyFileGenerator : public DependencyTracker {
-    std::vector<std::string> targets;
-    llvm::SetVector<std::string, std::vector<std::string>> paths;
-  public:
-    virtual void addDependency(StringRef file) override;
-
-    void addTarget(StringRef targetName);
-    void writeToStream(llvm::raw_ostream &os) const;
-  };
-}
-
-#endif