[MLIR] Add booleans to dense element list in LangRef

Add boolean values as a dense element, as tested in the MLIR parser:

```
  // CHECK: "splatBoolTensor"() {bar = dense<false> : tensor<i1>} : () -> ()
  "splatBoolTensor"(){bar = dense<false> : tensor<i1>} : () -> ()
```

https://github.com/llvm/llvm-project/blob/43d758b142bbdf94a1c55dc0950637ae74f825b9/mlir/test/IR/parser.mlir#L630-L631

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D122978