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

lib/Frontend/DependencyFileGenerator.cpp

@@ -0,0 +1,57 @@
+//===- DependencyFileGenerator.cpp - .d file generation -------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "swift/Frontend/DependencyFileGenerator.h"
+#include "llvm/ADT/SmallString.h"
+#include "llvm/Support/raw_ostream.h"
+
+using namespace swift;
+
+void DependencyFileGenerator::addDependency(StringRef file) {
+  paths.insert(file);
+}
+
+void DependencyFileGenerator::addTarget(StringRef targetName) {
+  if (!targetName.empty())
+    targets.push_back(targetName);
+}
+
+void DependencyFileGenerator::writeToStream(llvm::raw_ostream &os) const {
+  // Declare a helper for escaping file names for use in Makefiles.
+  llvm::SmallString<256> pathBuf;
+  auto escape = [&](StringRef raw) -> StringRef {
+    pathBuf.clear();
+    
+    static const char badChars[] = " $#:\n";
+    size_t prev = 0;
+    for (auto index = raw.find_first_of(badChars); index != StringRef::npos;
+         index = raw.find_first_of(badChars, index+1)) {
+      pathBuf.append(raw.slice(prev, index));
+      if (pathBuf[index] == '$')
+        pathBuf.push_back('$');
+      else
+        pathBuf.push_back('\\');
+      prev = index;
+    }
+    pathBuf.append(raw.substr(prev));
+    return pathBuf;
+  };
+
+  // FIXME: Xcode can't currently handle multiple targets in a single
+  // dependency line.
+  for (StringRef targetName : targets) {
+    os << targetName << " :";
+    for (StringRef path : paths)
+      os << ' ' << escape(path);
+    os << '\n';
+  }
+}