[clang] [clang-tools-extra] Add code completion for C++20 keywords. (PR #107982)

[Younan Zhang via cfe-commits]

================
@@ -2265,6 +2318,12 @@ AddOrdinaryNameResults(SemaCodeCompletion::ParserCompletionContext CCC,
       Results.AddResult(Result("template", CodeCompletionResult::RK_Keyword));
     }
 
+    if (SemaRef.getLangOpts().CPlusPlus20 &&
+        (CCC == SemaCodeCompletion::PCC_Template ||
+         CCC == SemaCodeCompletion::PCC_MemberTemplate)) {
+      Results.AddResult(Result("requires", CCP_Keyword));
+    }
----------------
zyn0217 wrote:

```suggestion
    if (SemaRef.getLangOpts().CPlusPlus20 &&
        (CCC == SemaCodeCompletion::PCC_Template ||
         CCC == SemaCodeCompletion::PCC_MemberTemplate))
      Results.AddResult(Result("requires", CCP_Keyword));
```

https://github.com/llvm/llvm-project/pull/107982