[clang] Surface error for plain return statement in coroutine earlier (PR #100985)

[Ilya Biryukov via cfe-commits]

================
@@ -694,6 +707,9 @@ bool Sema::ActOnCoroutineBodyStart(Scope *SC, SourceLocation KWLoc,
   auto *ScopeInfo = getCurFunction();
   assert(ScopeInfo->CoroutinePromise);
 
+  if (ScopeInfo->FirstCoroutineStmtLoc == KWLoc)
----------------
ilya-biryukov wrote:

NIT: add a comment explaning why we don't do this every time:

```cpp
// Avoid duplicate errors, report only on first keyword.
```

https://github.com/llvm/llvm-project/pull/100985