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

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()`.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D136669

Author: definelicht

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

@@ -1341,7 +1341,7 @@ def LLVM_LLVMFuncOp : LLVM_Op<"func", [
     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(); }
 
   }];