[llvm][MC] Remove [[nodiscard]] from WithMarkup constructor

This is causing a lot of warnings with older compilers and
errors for -Werror builders downstream.

I'm removing it until we can decide a good way to do this,
as it does seem important that this class not be discarded
given it controls formatting.

(cherry picked from commit 48987fb)

Author: DavidSpickett

llvm/include/llvm/MC/MCInstPrinter.h

@@ -97,7 +97,7 @@ class MCInstPrinter {
 
   class WithMarkup {
   public:
-    [[nodiscard]] WithMarkup(raw_ostream &OS, Markup M, bool EnableMarkup,
+    WithMarkup(raw_ostream &OS, Markup M, bool EnableMarkup,
                              bool EnableColor);
     ~WithMarkup();