Mark "target framework" as a variant dimension

[davkean]

One of many changes as we move to using the new IActiveConfigurationGroupService which returns all the configurations that we consider "active" for a multi-targeting scenarios (replacing IActiveConfiguredProjectsProvider).

• Mark "target framework" as a variant dimension
  This marks "target framework" dimension as a variant dimension; ie a configuration dimension that when present is effectively ignored when calculating "active configurations" returned by IActiveConfigurationGroupService:

For example, given the following multi-targeting project:

  -> All known project configurations:

      Configuration Platform    TargetFramework
      -------------------------------------------
              Debug |   AnyCPU  |           net45
              Debug |   AnyCPU  |           net46
            Release |   AnyCPU  |           net45
            Release |   AnyCPU  |           net46

  -> Active solution configuration:

              Debug |   AnyCPU  |           net45

  -> Target framework dimension is ignored, and active configurations returned by IActiveConfigurationGroupService:

              Debug |   AnyCPU  |           net45
              Debug |   AnyCPU  |           net46

Whereas, given the following non-multi-targeting project:

  -> All known project configurations:

      Configuration   Platform
      ------------------------
              Debug |   AnyCPU
            Release |   AnyCPU

  -> Active solution configuration:

              Debug |   AnyCPU

  -> Active configurations return active configurations returned by IActiveConfigurationGroupService:

              Debug |   AnyCPU

[davkean]

Still testing these bits, CPS is already consuming this - so adding this attribute will immediately take effect for the Error List. Now all errors/warnings from all design-time builds for a multi-targeting project will be added to it, instead of just the first "active" one.

[Pilchie]

LGTM - is the plan to merge this now and then follow up, or add more commits to this PR?

[davkean]

Don't plan on adding more commits to this PR - but am still testing this.

[davkean]

Okay was wrestling with this all day - finally got bits that work.

Before this change, given this project:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFrameworks>netcoreapp2.0;net45</TargetFrameworks>
  </PropertyGroup>

  <Target AfterTargets="Compile" Name="FooBar">

    <Warning Text="$(TargetFramework)" />
  </Target>
</Project>

The following would be output into the Error List after a design-time build:

Warning		netcoreapp2.0	ConsoleApp128	C:\Users\davkean\Source\Repos\ConsoleApp128\ConsoleApp128\ConsoleApp128.csproj	10	

After this change, the folllowing is now output to the Error List:

Warning		netcoreapp2.0	ConsoleApp128	C:\Users\davkean\Source\Repos\ConsoleApp128\ConsoleApp128\ConsoleApp128.csproj	10	
Warning		net45	ConsoleApp128	C:\Users\davkean\Source\Repos\ConsoleApp128\ConsoleApp128\ConsoleApp128.csproj	10	

[davkean]

This is ready for signoff.

[davkean]

@lifengl When are you planning on inserting this so that we can coordinate?

[jmarolf]

can we add a test for this please?

[davkean]

We don't have working VSI tests - how would you propose I write one?