IVsHierarchy Project GUID might not be initialized at the time AbstractLegacyproject reads it

For some legacy projects (e.g. legacy VB WPF app) the project system does not initialize Project GUID on IVsHierarchy before calling to Roslyn to construct the project.

Furthermore, it is unclear if the Project GUID passed to Roslyn is actually uniquely identifying the project or if it can be shared across multiple projects.

We need Project GUIDs that are stored on IVsHierarchy to uniquely identify projects created in Roslyn.

See also:
https://github.com/dotnet/roslyn/pull/36512
fixes [DevDiv 915798](https://dev.azure.com/devdiv/DevDiv/_workitems/edit/915798)
https://github.com/dotnet/roslyn/issues/36516

This issue is blocking work on VS SaaS.

Also tracked by [AB#981311](https://devdiv.visualstudio.com/0bdbc590-a062-4c3f-b0f6-9383f67865ee/_workitems/edit/981311)