[clang] [Clang] Warn about `[[noreturn]]` on coroutines (PR #127623)

[via cfe-commits]

================
@@ -3910,8 +3910,13 @@ StmtResult Sema::BuildReturnStmt(SourceLocation ReturnLoc, Expr *RetValExp,
     FnRetType = FD->getReturnType();
     if (FD->hasAttrs())
       Attrs = &FD->getAttrs();
-    if (FD->isNoReturn())
-      Diag(ReturnLoc, diag::warn_noreturn_function_has_return_expr) << FD;
+    if (FD->isNoReturn()) {
+      const FunctionScopeInfo *Scope = getEnclosingFunction();
+      if (Scope && Scope->isCoroutine())
+        Diag(ReturnLoc, diag::warn_noreturn_coroutine) << FD;
+      else
+        Diag(ReturnLoc, diag::warn_noreturn_function_has_return_expr) << FD;
+    }
----------------
Sirraide wrote:

```suggestion
    if (FD->isNoReturn() && !getCurFunction()->isCoroutine())
        Diag(ReturnLoc, diag::warn_noreturn_function_has_return_expr) << FD;
```

As is, this would have the effect of emitting a warning on every `return` statement; I don’t think that’s what we want given that the problem is not that we’re returning from a noreturn function, but rather that the function shouldn’t be noreturn to begin with, so I’d do something like this instead.

(also, `getCurFunction()` should be slightly faster than `getEnclosingFunction()` here and is equivalent because we’ve already handled capturing function scopes)

https://github.com/llvm/llvm-project/pull/127623