[clang] [Clang] Warn against unused parameters in C++ coroutines with `-Wunused-parameters` (PR #70567)

[Chuanqi Xu via cfe-commits]

================
@@ -3165,7 +3165,16 @@ class Sema final {
 
   /// Diagnose any unused parameters in the given sequence of
   /// ParmVarDecl pointers.
-  void DiagnoseUnusedParameters(ArrayRef<ParmVarDecl *> Parameters);
+  ///
+  /// Normally, we check if the parameter decls have the Referenced bit set.
+  /// C++ Coroutines, however, are a special case due to the existences of
+  /// parameter moves (See Sema::buildCoroutineParameterMoves), the parameters
+  /// are always referenced in coroutines. Therefore, in the case of coroutines,
+  /// CoroutineBodyRefs must be passed to correctly diagnose parameter usages
+  /// as written by the user.
+  void DiagnoseUnusedParameters(
+      ArrayRef<ParmVarDecl *> Parameters,
+      llvm::SmallSet<ParmVarDecl *, 4> *CoroutineBodyRefs = nullptr);
----------------
ChuanqiXu9 wrote:

Let's try to not polluting the interfaces as much as possible. Since most of the functions are not coroutines, it looks not good to pollute the interfaces.

Personally, I prefer the style:

```
if (the function is coroutine)
     DiagnoseUnusedParametersInCoroutines(...);
else
     DiagnoseUnusedParameters(...); // Original call.
```

Also it may be better to use `FunctionScopeInfo::CoroutineParameterMoves` than yet another DeclsVisitor. It is easy prone and hard to maintain.

https://github.com/llvm/llvm-project/pull/70567