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

[Ilya Biryukov via cfe-commits]

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@@ -694,6 +706,10 @@ bool Sema::ActOnCoroutineBodyStart(Scope *SC, SourceLocation KWLoc,
   auto *ScopeInfo = getCurFunction();
   assert(ScopeInfo->CoroutinePromise);
 
+  if (ScopeInfo->FirstCoroutineStmtLoc == KWLoc) {
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ilya-biryukov wrote:

NIT: remove braces from simple statements, see [LLVM Style Guide](https://llvm.org/docs/CodingStandards.html#don-t-use-braces-on-simple-single-statement-bodies-of-if-else-loop-statements)

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