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/AST/ModuleLoader.h

@@ -20,7 +20,6 @@
 #include "swift/AST/Identifier.h"
 #include "swift/Basic/LLVM.h"
 #include "swift/Basic/SourceLoc.h"
-#include "llvm/ADT/IntrusiveRefCntPtr.h"
 
 namespace swift {
 
@@ -29,10 +28,28 @@ class NominalTypeDecl;
 
 enum class KnownProtocolKind : uint8_t;
 
+class DependencyTracker {
+  virtual void anchor();
+protected:
+  DependencyTracker() = default;
+public:
+  virtual ~DependencyTracker() = default;
+  virtual void addDependency(StringRef file) {}
+};
+
 /// \brief Abstract interface that loads named modules into the AST.
 class ModuleLoader {
+  DependencyTracker * const dependencyTracker;
   virtual void anchor();
 
+protected:
+  ModuleLoader(DependencyTracker *tracker) : dependencyTracker(tracker) {}
+
+  void addDependency(StringRef file) {
+    if (dependencyTracker)
+      dependencyTracker->addDependency(file);
+  }
+
 public:
   virtual ~ModuleLoader() = default;