Initial set of changes to add a new 'swift_driver' executable.

- Added a couple of new targets:
  - libswiftDriver, which contains most of the driver implementation
  - swift_driver, which produces the actual executable

- Added centralized version information into libswiftBasic.

- Added a new "Driver Design & Internals" document, which currently describes
  the high-level design of the Swift driver.

- Implemented an early version of the functionality of the driver, including
  versions of the Parse, Pipeline, Bind, Translate, and Execute driver stages.
  Parse, Pipeline, and Bind are largely implemented; Translate and Execute are
  early placeholders. (Translate produces "swift_driver --version" and "ld -v"
  commands, while Execute performs all subtasks sequentially, rather than in
  parallel.)

This is just the starting point for the Swift driver. Tests for the existing
behavior are forthcoming.

Swift SVN r10933

lib/Driver/Job.cpp

@@ -0,0 +1,96 @@
+//===--- Job.cpp - Command to Execute -------------------------------------===//
+//
+// 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/Driver/Job.h"
+
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/Support/Program.h"
+
+using namespace swift;
+using namespace swift::driver;
+
+JobList::~JobList() {
+  if (OwnsJobs) {
+    llvm::DeleteContainerPointers(Jobs);
+  }
+}
+
+bool JobList::needsToRun() {
+  for (Job *J : *this) {
+    if (J->needsToRun()) {
+      return true;
+    }
+  }
+
+  // None of the jobs in this list need to run, so the list doesn't need to run.
+  return false;
+}
+
+int JobList::run() {
+  int result = 0;
+  for (Job *J : *this) {
+    if (J->needsToRun() && ((result = J->run()) != 0)) {
+      return result;
+    }
+  }
+  return result;
+}
+
+void JobList::clear() {
+  if (OwnsJobs) {
+    llvm::DeleteContainerPointers(Jobs);
+  } else {
+    // We don't own the pointers, so just clear the list.
+    Jobs.clear();
+  }
+}
+
+bool Command::needsToRun() {
+  if (Inputs->empty()) {
+    // If a command has no inputs, it must be run.
+    // TODO: refine this check, though possibly only in subclasses of Command.
+    return true;
+  }
+
+  // If this command has inputs, then we fall back to whether or not they need
+  // to run.
+  return Inputs->needsToRun();
+}
+
+int Command::run() {
+  int result = 0;
+  for (Job *J : *Inputs) {
+    if (J->needsToRun()) {
+      result = J->run();
+      if (result != 0) {
+        // This Job failed (returned a non-zero result code), so stop here.
+        return result;
+      }
+    }
+  }
+
+  return execute(nullptr, nullptr, nullptr);
+}
+
+int Command::execute(const StringRef **Redirects, std::string *ErrMsg,
+                     bool *ExecutionFailed) const {
+  // TODO: this needs to be replaced with something which we can dispatch
+  // multiple at a time.
+
+  SmallVector<const char *, 128> Argv;
+  Argv.push_back(Executable);
+  Argv.append(Arguments.begin(), Arguments.end());
+  Argv.push_back(0);
+
+  return llvm::sys::ExecuteAndWait(Executable, Argv.data(), 0,
+                                   Redirects, 0, 0, ErrMsg, ExecutionFailed);
+}