Build Cecil repo in VMR

[MichaelSimons]

With the work to merge linker into the runtime repo, the cecil repo will no longer be a submodule and instead will be a standalone repo. Once the runtime changes flow in to add a dependency on the cecil repo, a source-build cecil project will need to define in order to get it to build.

[tlakollo]

The dotnet cecil project already has source build information being produced and runtime adds the SourceBuild tag into that depedency, is anything else required?

[MichaelSimons]

The dotnet cecil project already has source build information being produced and runtime adds the SourceBuild tag into that depedency, is anything else required?

Yes, there has to be a way in the full product source-build to trigger the cecil repo build. This is done via the repo-projects.

[MichaelSimons]

@mthalman - can you add this project and validate cecil is prebuilt clean? It is dependent on dotnet/installer#15389 flowing it but you could start work now based on that branch.