We now compile against the reference assembly in all places where we were compiling against the mono/coreclr System.Private.CoreLib.dll implementation assembly before.
The new reference assembly consumes sources from the existing contracts to avoid checking in a generated version of SPC.dll (this would add ~20k lines of .cs which is mostly duplicated with System.Runtime.cs)
Since a few contracts have only partially moved types to SPC we wrap contract types with `#if !BUILDING_CORELIB_REFERENCE` so we can hide them when compiling the SPC reference assembly. This needed a few GenAPI changes which are implemented here: https://github.com/dotnet/arcade/pull/10003.
Note that this means that the types which live in CoreLib are moved to the end of the ref .cs file which causes a GitHub diff to show up, but it is actually a no-op.
Regenerating the ref .cs files works the same way as before, by running the `GenerateReferenceAssemblySource` target in the contract's src\ folder.
Fixes https://github.com/dotnet/runtime/issues/67660
Co-authored-by: Viktor Hofer <viktor.hofer@microsoft.com>
