[clang] [Clang][Coroutines] Properly emit EH code for initial suspend `await_resume` (PR #73073)

[Yuxuan Chen via cfe-commits]

https://github.com/yuxuanchen1997 created https://github.com/llvm/llvm-project/pull/73073

This change aims to fix an ICE in issue https://github.com/llvm/llvm-project/issues/63803

The crash happens in `ExitCXXTryStmt` because `EmitAnyExpr()` adds additional cleanup to the `EHScopeStack`. This messes up the assumption in `ExitCXXTryStmt` that the top of the stack should be a `EHCatchScope`. 

However, since we never read a value returned from `await_resume()` of an init suspend, we can skip the part that builds this `RValue`.

>From f782f36c42f9bc1246837bf7ff2142919794580b Mon Sep 17 00:00:00 2001
From: Yuxuan Chen <ych at meta.com>
Date: Tue, 21 Nov 2023 19:06:31 -0800
Subject: [PATCH] [Clang][coro] Fix crash on emitting init suspend if the
 return type of await_resume() is not trivially destructible

---
 clang/lib/CodeGen/CGCoroutine.cpp | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/clang/lib/CodeGen/CGCoroutine.cpp b/clang/lib/CodeGen/CGCoroutine.cpp
index 7e449d5af3423cf..aaf122c0f83bc47 100644
--- a/clang/lib/CodeGen/CGCoroutine.cpp
+++ b/clang/lib/CodeGen/CGCoroutine.cpp
@@ -245,6 +245,15 @@ static LValueOrRValue emitSuspendExpression(CodeGenFunction &CGF, CGCoroData &Co
                                          FPOptionsOverride(), Loc, Loc);
     TryStmt = CXXTryStmt::Create(CGF.getContext(), Loc, TryBody, Catch);
     CGF.EnterCXXTryStmt(*TryStmt);
+    CGF.EmitStmt(TryBody);
+    // We don't use EmitCXXTryStmt here. We need to store to ResumeEHVar that
+    // doesn't exist in the body.
+    Builder.CreateFlagStore(false, Coro.ResumeEHVar);
+    CGF.ExitCXXTryStmt(*TryStmt);
+    LValueOrRValue Res;
+    // We are not supposed to obtain the value from init suspend await_resume().
+    Res.RV = RValue::getIgnored();
+    return Res;
   }
 
   LValueOrRValue Res;
@@ -253,11 +262,6 @@ static LValueOrRValue emitSuspendExpression(CodeGenFunction &CGF, CGCoroData &Co
   else
     Res.RV = CGF.EmitAnyExpr(S.getResumeExpr(), aggSlot, ignoreResult);
 
-  if (TryStmt) {
-    Builder.CreateFlagStore(false, Coro.ResumeEHVar);
-    CGF.ExitCXXTryStmt(*TryStmt);
-  }
-
   return Res;
 }