[HW] Roundtrip test fail for !hw.union 

```bash
$ circt-opt test/Conversion/ExportVerilog/hw-dialect.mlir --test-apply-lowering-options='options=emittedLineLength=100,emitBindComments' -export-verilog -verify-diagnostics -o a.mlir
$ circt-opt a.mlir
a.mlir:788:56: error: expected non-function type
    %1 = sv.wire {hw.verilogName = "_GEN"} : !hw.inout<union<c: i1>>
```