[clang] [Sema] Add code completion for if constexpr (PR #124315)

[via cfe-commits]

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@@ -6749,6 +6749,21 @@ void SemaCodeCompletion::CodeCompleteInitializer(Scope *S, Decl *D) {
   CodeCompleteExpression(S, Data);
 }
 
+void SemaCodeCompletion::CodeCompleteIfConstExpr(Scope *S) const {
+  ResultBuilder Results(SemaRef, CodeCompleter->getAllocator(),
+                        CodeCompleter->getCodeCompletionTUInfo(),
+                        CodeCompletionContext::CCC_SymbolOrNewName);
+  Results.EnterNewScope();
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FantasqueX wrote:

I'm not familiar with this but There is a `Results.EnterNewScope();` following almost every `ResultBuilder`. So, I think I should add one as well.

https://github.com/llvm/llvm-project/pull/124315