Add support for dependency file generation with -emit-dependencies.

This performs very conservative dependency generation for each compile task
within a full compilation. Any source file, swiftmodule, or Objective-C
header file that is /touched/ gets added to the dependencies list, which
is written out on a per-input basis at the end of compilation.

This does /not/ handle dependencies for the aggregated swiftmodule, swiftdoc,
generated header, or linked binary. This is just the minimum needed to get
Xcode to recognize what needs to be rebuilt when a header or Swift source
file changes. We can revisit this later.

This finishes <rdar://problem/14899639> for now.

Swift SVN r18045

include/swift/Frontend/DependencyFileGenerator.h

@@ -0,0 +1,35 @@
+//===- 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>,
+                    llvm::StringSet<>> paths;
+  public:
+    virtual void addDependency(StringRef file) override;
+
+    void addTarget(StringRef targetName);
+    void writeToStream(llvm::raw_ostream &os) const;
+  };
+}
+
+#endif