[utils] Add a util to help printing with colors. NFC (#4665)

* [utils] Add a util to help printing with colors. NFC

* Update macro guard for consistency.

* add some doc-comments.

Author: nkcsgexi

include/swift/Basic/ColorUtils.h

@@ -0,0 +1,47 @@
+//===--- ColorUtils.h - -----------------------------------------*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2016 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
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines an 'OSColor' class for helping printing colorful outputs
+// to the terminal.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_BASIC_COLORUTILS_H
+#define SWIFT_BASIC_COLORUTILS_H
+
+#include "llvm/Support/raw_ostream.h"
+
+namespace swift {
+
+/// RAII class for setting a color for a raw_ostream and resetting when it goes
+/// out-of-scope.
+class OSColor {
+  llvm::raw_ostream &OS;
+  bool HasColors;
+public:
+  OSColor(llvm::raw_ostream &OS, llvm::raw_ostream::Colors Color) : OS(OS) {
+    HasColors = OS.has_colors();
+    if (HasColors)
+      OS.changeColor(Color);
+  }
+  ~OSColor() {
+    if (HasColors)
+      OS.resetColor();
+  }
+
+  OSColor &operator<<(char C) { OS << C; return *this; }
+  OSColor &operator<<(llvm::StringRef Str) { OS << Str; return *this; }
+};
+
+} // namespace swift
+
+#endif // SWIFT_BASIC_COLORUTILS_H