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/Serialization/SerializedModuleLoader.h

@@ -56,14 +56,15 @@ private:
   using LoadedModulePair = std::pair<std::unique_ptr<ModuleFile>, unsigned>;
   std::vector<LoadedModulePair> LoadedModuleFiles;
 
-  explicit SerializedModuleLoader(ASTContext &ctx);
+  explicit SerializedModuleLoader(ASTContext &ctx, DependencyTracker *tracker);
 
 public:
   /// \brief Create a new importer that can load serialized Swift modules
   /// into the given ASTContext.
-  static std::unique_ptr<SerializedModuleLoader> create(ASTContext &ctx) {
+  static std::unique_ptr<SerializedModuleLoader>
+  create(ASTContext &ctx, DependencyTracker *tracker = nullptr) {
     return std::unique_ptr<SerializedModuleLoader>{
-      new SerializedModuleLoader(ctx)
+      new SerializedModuleLoader(ctx, tracker)
     };
   }