Add an experimental API for testing incremental steps.

[jkoritzinsky]

Add a .NET 6.0 test target that references Roslyn 4.4 so we have a test suite that references Roslyn 4.X.

Introduce a new API to enable developers to define post-initial-run solution transforms to test incrementality.

Add a Humanizer reference at the same version as Roslyn's dependency to improve the error message wording of the new validation section (can be removed if we so desire).

Note

Notes from @sharwell on specific tests we believe would be valuable. These tests could be provided by the API in this pull request, or through some other means.

• Ability to verify that a change which did not impact outputs only executed specific incremental steps. Since it's easy (per @jkoritzinsky) to unintentionally break equality comparisons for model objects, there is a likelihood of regression in the incremental evaluation due to equivalent items returning not-equal.
• Ability to verify that a change which only impacted a subset of outputs only executed specific incremental steps that serve as inputs to other outputs which did not change.
• Ability to test that a source generator did not include Compilation as part of its incremental state. In other words, after running source generators on a compilation and still holding the GeneratorDriver instance, the Compilation passed to it should be eligible for garbage collection.

[sharwell]

📝 I would prefer to remove this dependency before merge, but that's further down the line. I'm mostly focusing on the API itself right now.

[sharwell]

💡 I think we can remove this in favor of changing the type of IncrementalGeneratorStepStates to a new type derived from Dictionary<Type, IncrementalGeneratorExpectedState> that provides a new indexer that creates the value on demand. In practice in means you'd have to type this, but it seems acceptable:

IncrementalGeneratorStepStates =
{
    [typeof(TSourceGenerator)] =
    {
        ExpectedStepStates =
        {
            // ...
        },
    },
},

[sharwell]

❔ Will the driver recognize cases where a file did not change? For example, consider the case of two additional files. If you change the content of just one of them, will it treat the other one as unchanged?

[Youssef1313]

Isn't it okay to have empty sources but non-empty generated sources?