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

[Younan Zhang via cfe-commits]

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@@ -152,6 +152,7 @@ class SemaCodeCompletion : public SemaBase {
   void CodeCompleteDesignator(const QualType BaseType,
                               llvm::ArrayRef<Expr *> InitExprs,
                               const Designation &D);
+  void CodeCompleteIfConstExpr(Scope *S) const;
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zyn0217 wrote:

```suggestion
  void CodeCompleteIfConstexpr(Scope *S) const;
```

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