[mlir][llvm] Don't return a dangling reference in getCallableResults().

[definelicht]

Summary

Use the getReturnTypes() API (which returns an ArrayRef<Type>)
rather than the getReturnType() API (which returns a Type) to avoid
returning a dangling reference in LLVMFuncOp::getCallableResults().

mlir/include/mlir/Dialect/LLVMIR/LLVMOps.td

let extraClassDeclaration = [{
 
     /// Returns the result types of this function.
     ArrayRef<Type> getResultTypes() { return getFunctionType().getReturnTypes(); }
 
     /// Returns the callable region, which is the function body.
     Region *getCallableRegion() { return &getBody(); }
 
     /// Returns the callable result type, which is the function return type.
-    ArrayRef<Type> getCallableResults() { return getFunctionType().getReturnType(); }
+    ArrayRef<Type> getCallableResults() { return getFunctionType().getReturnTypes(); }
 
   }];
 
   let hasCustomAssemblyFormat = 1;
   let hasVerifier = 1;
   let hasRegionVerifier = 1;
 }