Remove some FIXMEs that no longer apply; NFC

Noticed these while doing a review on changes in the area, but C23
added support for nodiscard with a message, so it's not an extension
we need to diagnose.

Author: AaronBallman

clang/lib/Sema/SemaDeclAttr.cpp

@@ -2912,18 +2912,14 @@ static void handleWarnUnusedResult(Sema &S, Decl *D, const ParsedAttr &AL) {
 
     // If this is spelled as the standard C++17 attribute, but not in C++17,
     // warn about using it as an extension. If there are attribute arguments,
-    // then claim it's a C++20 extension instead.
-    // FIXME: If WG14 does not seem likely to adopt the same feature, add an
-    // extension warning for C23 mode.
+    // then claim it's a C++20 extension instead. C23 supports this attribute
+    // with the message; no extension warning is needed there beyond the one
+    // already issued for accepting attributes in older modes.
     const LangOptions &LO = S.getLangOpts();
     if (AL.getNumArgs() == 1) {
       if (LO.CPlusPlus && !LO.CPlusPlus20)
         S.Diag(AL.getLoc(), diag::ext_cxx20_attr) << AL;
 
-      // Since this is spelled [[nodiscard]], get the optional string
-      // literal. If in C++ mode, but not in C++20 mode, diagnose as an
-      // extension.
-      // FIXME: C23 should support this feature as well, even as an extension.
       if (!S.checkStringLiteralArgumentAttr(AL, 0, Str, nullptr))
         return;
     } else if (LO.CPlusPlus && !LO.CPlusPlus17)