[llvm] Add LLVM_CTOR_NODISCARD to WithColor

Similarly to WithMarkup, add LLVM_CTOR_NODISCARD to the WithColor
constructor.

(cherry picked from commit 511b4dd)

Author: JDevlieghere

llvm/include/llvm/Support/WithColor.h

@@ -9,6 +9,7 @@
 #ifndef LLVM_SUPPORT_WITHCOLOR_H
 #define LLVM_SUPPORT_WITHCOLOR_H
 
+#include "llvm/Support/Compiler.h"
 #include "llvm/Support/raw_ostream.h"
 
 namespace llvm {
@@ -58,19 +59,18 @@ class WithColor {
   /// @param OS The output stream
   /// @param S Symbolic name for syntax element to color
   /// @param Mode Enable, disable or compute whether to use colors.
-  WithColor(raw_ostream &OS, HighlightColor S,
-            ColorMode Mode = ColorMode::Auto);
+  LLVM_CTOR_NODISCARD WithColor(raw_ostream &OS, HighlightColor S,
+                                ColorMode Mode = ColorMode::Auto);
   /// To be used like this: WithColor(OS, raw_ostream::BLACK) << "text";
   /// @param OS The output stream
   /// @param Color ANSI color to use, the special SAVEDCOLOR can be used to
   /// change only the bold attribute, and keep colors untouched
   /// @param Bold Bold/brighter text, default false
   /// @param BG If true, change the background, default: change foreground
   /// @param Mode Enable, disable or compute whether to use colors.
-  WithColor(raw_ostream &OS,
-            raw_ostream::Colors Color = raw_ostream::SAVEDCOLOR,
-            bool Bold = false, bool BG = false,
-            ColorMode Mode = ColorMode::Auto)
+  LLVM_CTOR_NODISCARD WithColor(
+      raw_ostream &OS, raw_ostream::Colors Color = raw_ostream::SAVEDCOLOR,
+      bool Bold = false, bool BG = false, ColorMode Mode = ColorMode::Auto)
       : OS(OS), Mode(Mode) {
     changeColor(Color, Bold, BG);
   }