[program-gen] Fix enum resolution from types of the form Union[string, Enum] and emit fully qualified enum cases

[Zaid-Ajaj]

Description

This PR improves enum type resolution from strings. When we try to resolve Union[string, Enum] for a string expression, we choose string because it is the more general type since not every string is assignable to Enum. However, here we spacial case strings that are actually part of that Enum.

The result is that pcl.LowerConversion will choose Enum from Union[string, Enum] when the value of the input string is compatible with the enum. This greatly improves program-gen for all of typescript, python, csharp and go which now will emit the fully qualified enum cases instead of emitting strings.

Closes pulumi/pulumi-dotnet#41 which is supposed to be a duplicate of pulumi/pulumi-azure-native#2616 but that is not the case (the former is about unions of objects, the latter is unions of enums and strings)

Checklist

• I have run make tidy to update any new dependencies
• I have run make lint to verify my code passes the lint check
  • I have formatted my code using gofumpt

• I have added tests that prove my fix is effective or that my feature works

• I have run make changelog and committed the changelog/pending/<file> documenting my change

• Yes, there are changes in this PR that warrants bumping the Pulumi Cloud API version

[pulumi-bot]

Changelog

[uncommitted] (2024-03-15)

Bug Fixes

• [programgen/{dotnet,go,nodejs,python}] Fix enum resolution from types of the form union[string, enum]
  #15696