Enable unified build
@wtgodbe First draft of support for UB in aspnetcore. The goals here are that aspentcore builds a vertical (generally Target Arch+OS) in the VMR. Source build infra is used, but switches aren't.
There are definitely some rough edges here and further work is required. I've annotated the PR.
Covers https://github.com/dotnet/source-build/issues/4022 and https://github.com/dotnet/aspnetcore/issues/43637 with caveats.
Testing this in a CI build now
I think this is ready to go. May need additional tweaks but this should allow us to enable aspnet
Ready to go