[pigeon] Improve C++ field handling

[stuartmorgan-g]

This implements non-nullable field support in the C++ generator, and fixes some cases where the C++ generation for nullable fields was assuming non-nullable values.

As part of this:

• Factors data class generation out into helper methods.
• Adds isNullable to HostDatatype to make it easier to track nullability across translation from Dart to host datatypes.
• Ports the existing nullable and non-nullable field native unit tests from ObjC to C++, giving native unit test coverage of most of the changes here.
  • Adds a hook in the C++ generator to generate a test friend for a special namespace to facilitate these tests.
• Enables building the non-nullable field file.
• Adjusts the ivar, setter parameter, and getter return types to match nullability expectations.
  • Adds a second convenience setter for nullable types that takes non-nullable values, to facilitate use of literals when constructing objects on the C++ side.

Fixes flutter/flutter#104278

Pre-launch Checklist

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• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
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• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

[stuartmorgan-g]

I moved these without changes as the very first commit, so excluding the first commit from the review will make it easier to see the substantive changes.

[stuartmorgan-g]

I suspect that there are at least some bugs similar to the ones I fixed for field types in other places, but I wanted to limit the scope of this PR. We should get more of the native tests online in follow-ups to flush out other bugs.

[azchohfi]

Yes. This PR removes fields from being unique_ptrs, so the use of this method should be greatly reduced here.

[azchohfi]

Yes. This PR removes fields from being unique_ptrs, so the use of this method should be greatly reduced here.

[azchohfi]

@stuartmorgan everything is working fine on my end.
Only one question:
The setters are no longer being generated with const args. Is that expected, or was it an oversight?

[stuartmorgan-g]

The setters are no longer being generated with const args. Is that expected, or was it an oversight?

I assume you mean the non-null POD case? For the declaration it doesn't do anything (https://abseil.io/tips/109), and for the definition it seemed like pointless boilerplate in the generated code (not to mention extra work in the generator) given that it's a one-line implementation, so there's not really any room to accidentally modify the parameter.

[azchohfi]

I assume you mean the non-null POD case? For the declaration it doesn't do anything (https://abseil.io/tips/109), and for the definition it seemed like pointless boilerplate in the generated code (not to mention extra work in the generator) given that it's a one-line implementation, so there's not really any room to accidentally modify the parameter.

I was mostly thinking about compiler optimizations due to it being const, but I don't think it would change much, if at all.