If we were in a situation like in the regression test, we would devirtualize the `GrabObject` call to `SomeUnallocatedClass.GrabObject`. But because `SomeUnallocatedClass` was never allocated, the scanner didn't scan it, and bad things would happen.

Prevent devirtualizing into types that were not seen as allocated.

This is not a real issue for non-generic (non-shareable) types because the tentative instance method optimization generates throwing bodies for these. But tentative method optimization doesn't run for shared generics:

https://github.com/dotnet/runtime/blob/4cbe6f99d23e04c56a89251d49de1b0f14000427/src/coreclr/tools/Common/Compiler/MethodExtensions.cs#L115

This was rare enough that we haven't seen it until I did #73683 and there was one useless constructor that we stopped generating and triggered this.

This also includes what is essentially a rollback of https://github.com/dotnet/runtimelab/pull/1700. This should have been rolled back with https://github.com/dotnet/runtime/pull/66145 but I forgot we had this. It was not needed.

* Update tests.proj