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

[Younan Zhang via cfe-commits]

================
@@ -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();
+
+  Results.AddResult(CodeCompletionResult("constexpr"));
----------------
zyn0217 wrote:

And we might want to offer `if consteval` as well

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