SetPlatform Negotiation: Allow MSBuild GetTargetFrameworks call when SetTargetFramework already set

[benvillalobos]

Context

Fix potential issue where an already-set 'SetTargetFramework' metadata on a ProjectReference item would skip logic required for SetPlatform negotiation.

When SkipGetTargetFrameworkProperties is true, it skips the MSBuild call to GetTargetFrameworks. I modified a condition to ensure it wouldn't be set to true when opted into the feature, but just below that was this:

    <ItemGroup>
      <_MSBuildProjectReferenceExistent Condition="'%(_MSBuildProjectReferenceExistent.SetTargetFramework)' != ''">
        <SkipGetTargetFrameworkProperties>true</SkipGetTargetFrameworkProperties>
      </_MSBuildProjectReferenceExistent>
    </ItemGroup>

So if we skipped setting SkipGetTargetFrameworkProperties to true, it would still be set to true if SetTargetFramework was set. This would skip the call to GetTargetFrameworks, which would prevent the proper metadata being added to the item before GetCompatiblePlatform is called, likely resulting in a warning and the project being built with no metadata

Changes Made

Added a condition not to set SkipGetTargetFrameworkProperties when opted into platform negotiation.

Testing

Not tested yet.

Notes

One of those "wake up in the middle of the night and this hits you" moments.

[benvillalobos]

Tested locally on a managed project reference to an unmanaged project.

This is what happens when SetTargetFramework is set already

With the fix:

Notice SetTargetFramework is set on the project item in the MSBuild task call and the task call still happens.

The result: Even when SetTargetFramework is set, when we're opted into platform negotiation we still get the GetTargetFrameworks MSBuild call that platform negotiation needs.

[Forgind]

Sounds reasonable