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

[Ilya Biryukov via cfe-commits]

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@@ -684,6 +684,18 @@ bool Sema::checkFinalSuspendNoThrow(const Stmt *FinalSuspend) {
   return ThrowingDecls.empty();
 }
 
+// [stmt.return.coroutine]p1:
+//   A coroutine shall not enclose a return statement ([stmt.return]).
+static void checkReturnStmtInCoroutine(Sema &S, FunctionScopeInfo *FSI) {
+  if (FSI && FSI->FirstReturnLoc.isValid()) {
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ilya-biryukov wrote:

NIT: negate the condition to reduce nesting, see LLVM style guide on [using early exits](https://llvm.org/docs/CodingStandards.html#use-early-exits-and-continue-to-simplify-code)

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