[driver] Added support for executing a single Command without a TaskQueue.

Added swift::ExecuteInPlace(), which on Unix acts as a wrapper for execv()
and execve(). On other platforms, swift::ExecuteInPlace() is a wrapper for
llvm::sys::ExecuteAndWait(), so callers must be prepared for ExecuteInPlace()
to return in a non-error situation.

Added support in Compilation::performJobs() to detect that the Compilation has
exactly one Command to run. If that's the case, and buffered output isn't
required, execute that Command using swift::ExecuteInPlace() (instead of
creating a TaskQueue, which may unconditionally buffer output).

This change will allow the driver to invoke the frontend's REPL and immediate
modes without buffering output (and, on Unix, without a separate process being
spawned).

Also updated test/Driver/basic.swift to emit output with these changes in place.

Swift SVN r12544

lib/Basic/Program.cpp

@@ -0,0 +1,38 @@
+//===-- Program.cpp - Implement OS Program Concept --------------*- C++ -*-===//
+//
+// 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/Basic/Program.h"
+
+#include "llvm/Config/config.h"
+#include "llvm/Support/Program.h"
+
+#if LLVM_ON_UNIX
+#if HAVE_UNISTD_H
+#include <unistd.h>
+#endif
+#endif
+
+int swift::ExecuteInPlace(const char *Program, const char **args,
+                          const char **env) {
+#if LLVM_ON_UNIX
+  int result;
+  if (env)
+    result = execve(Program, const_cast<char **>(args),
+                    const_cast<char **>(env));
+  else
+    result = execv(Program, const_cast<char **>(args));
+
+  return result;
+#else
+  return llvm::sys::ExecuteAndWait(Program, args, env);
+#endif
+}