[HW] Unkown location attributes on HWModule ports missing in serialized IR

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Follow-up on #6388/#6416

The merged PR hotfixes the crash during compilation, but there remain inconsistencies in the serialized output format.
Pushing the example of #6388 through `circt-opt --mlir-print-debuginfo --mlir-print-local-scope` prints:

```
module {
hw.module @test(in %I : i1, out O : i1) {
hw.output %I : i1 loc("":3:9)
} loc("":2:5)
} loc("":1:1)
```

The explicitly unknown locations are missing. Parsing and printing this output again produces:

```
module {
hw.module @test(in %I : i1 loc("":2:22), out O : i1 loc("":2:35)) {
hw.output %I : i1 loc("":3:9)
} loc("":2:5)
} loc("":1:1)
```

While this could likely be fixed in `module_like_impl::printModuleSignatureNew`, this indicates that there is a semantic difference between having no location vs. having an unknown location. So, removing unknown location attributes from ports and reconstructing them on demand could be problematic in general. There are also concerns regarding performance (see #6257).

Start with the #6388 example and reproduce its output using circt-opt --mlir-print-debuginfo --mlir-print-local-scope. Inspect module_like_impl::printModuleSignatureNew and the handling of unknown locations on HWModule ports, then compare parse/print round trips. Done means explicitly unknown port locations are preserved consistently without losing the distinction between missing and unknown locations.

Written by the indexing model from the issue text.