[clang] [Clang] Preserve coroutine parameter referenced state (PR #70973)

[Chuanqi Xu via cfe-commits]

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@@ -1965,9 +1965,15 @@ bool Sema::buildCoroutineParameterMoves(SourceLocation Loc) {
     if (PD->getType()->isDependentType())
       continue;
 
+    // Preserve the referenced state for unused parameter diagnostics.
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ChuanqiXu9 wrote:

Readers can understand what is going on here. But they may be confused about the motivation. Let's add a comment to explain the reasons. I mean, something like, if we don't do so, we can't diagnose the unused parameter things.

https://github.com/llvm/llvm-project/pull/70973