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/Sema/SourceLoader.cpp

@@ -81,6 +81,8 @@ Module *SourceLoader::loadModule(SourceLoc importLoc,
     return nullptr;
   }
 
+  addDependency(inputFile->getBufferIdentifier());
+
   // Turn off debugging while parsing other modules.
   llvm::SaveAndRestore<bool> turnOffDebug(Ctx.LangOpts.DebugConstraintSolver,
                                           false);