[FIRRTL][LowerToHW] Zero-width signals with inner symbol are silently deleted, breaking users

Consider this MLIR example:

```
module {
firrtl.circuit "OMIRField" {
firrtl.module @OMIRField(in %x_b: !firrtl.uint<0>, out %y_b: !firrtl.uint<0>) {
%n_b = firrtl.node sym @omir_sym %x_b : !firrtl.uint<0>
firrtl.strictconnect %y_b, %n_b : !firrtl.uint<0>
}
}
sv.verbatim "Testing {{0}}" {symbols = [#hw.innerNameRef<@OMIRField::@omir_sym>]}
}
```

Feeding through LowerToHW: `circt-opt --lower-firrtl-to-hw` produces:

```
module {
sv.verbatim "Testing {{0}}" {symbols = [#hw.innerNameRef<@OMIRField::@omir_sym>]}
hw.module @OMIRField() {
hw.output
}
}
```

And if run through `firtool` directly, produces:
```
omir_zw_field_issue.mlir:8:3: error: cannot get name for symbol #hw.innerNameRef<@OMIRField::@omir_sym>
sv.verbatim "Testing {{0}}" {symbols = [#hw.innerNameRef<@OMIRField::@omir_sym>]}
^
omir_zw_field_issue.mlir:8:3: note: see current operation: "sv.verbatim"() {format_string = "Testing {{0}}", symbols = [#hw.innerNameRef<@OMIRField::@omir_sym>]} : () -> ()
// Generated by CIRCT 1.46.0g20230714_98d0bf5
Testing // omir_zw_field_issue.mlir:8:3
module OMIRField(); // omir_zw_field_issue.mlir:3:5
endmodule
```

The non-failing error isn't great and should be fixed (cc #4770 for similar in adjacent code).
Also, this should be caught by the verifier in HW but that support isn't in place yet (cc #3526).

---

Moving the symbol to a port produces an error, modified input:

```
module {
firrtl.circuit "OMIRField" {
firrtl.module @OMIRField(in %x_b: !firrtl.uint<0> sym @omir_sym, out %y_b: !firrtl.uint<0>) {
%n_b = firrtl.node %x_b : !firrtl.uint<0>
firrtl.strictconnect %y_b, %n_b : !firrtl.uint<0>
}
}
sv.verbatim "Testing {{0}}" {symbols = [#hw.innerNameRef<@OMIRField::@omir_sym>]}
}
```

Error:
```
omir_zw_field_issue.mlir:3:33: error: zero width port "x_b" is referenced by name [#hw] (e.g. in an XMR) but must be removed
firrtl.module @OMIRField(in %x_b: !firrtl.uint<0> sym @omir_sym, out %y_b: !firrtl.uint<0>) {
^
```

Start with the supplied MLIR reproducer and the --lower-firrtl-to-hw entry point, then trace how zero-width signals with inner symbols are handled. Compare the node and port cases shown in the issue, and review the related concerns in #4770 and #3526. Done should prevent referenced symbols from being silently deleted and provide the expected diagnostic or verifier behavior.

Written by the indexing model from the issue text.