[Coroutines] Fix -Wunused-variable in CoroFrame.cpp (NFC)

/llvm-project/llvm/lib/Transforms/Coroutines/CoroFrame.cpp:1124:15:
error: unused variable 'PromiseAlloca' [-Werror,-Wunused-variable]
  AllocaInst *PromiseAlloca = Shape.getPromiseAlloca();
              ^
1 error generated.

Author: DamonFool

llvm/lib/Transforms/Coroutines/CoroFrame.cpp

@@ -1121,8 +1121,7 @@ static void buildFrameDebugInfo(Function &F, coro::Shape &Shape,
 
   DIBuilder DBuilder(*F.getParent(), /*AllowUnresolved*/ false);
 
-  AllocaInst *PromiseAlloca = Shape.getPromiseAlloca();
-  assert(PromiseAlloca &&
+  assert(Shape.getPromiseAlloca() &&
          "Coroutine with switch ABI should own Promise alloca");
 
   DIFile *DFile = DIS->getFile();