Don't generate Unmanaged-to-Managed stubs with diagnostics

[jkoritzinsky]

This PR disables our "partial generation" feature for the unmanaged->managed stubs. The partial generation generates many invalid [UnmanagedCallersOnly] method signatures, which causes compiler errors. By disabling the generation, we ensure that the source-generated code doesn't produce any compiler errors itself.

Fixes #82581

[ghost]

Tagging subscribers to this area: @dotnet/interop-contrib
See info in area-owners.md if you want to be subscribed.

Issue Details

---

This PR disables our "partial generation" feature for the unmanaged->managed stubs. The partial generation generates many invalid [UnmanagedCallersOnly] method signatures, which causes compiler errors. By disabling the generation, we ensure that the source-generated code doesn't produce any compiler errors itself.

Fixes #82581

Author:	jkoritzinsky
Assignees:	-
Labels:	area-System.Runtime.InteropServices, source-generator
Milestone:	8.0.0

[jtschuster]

Should we change all the warnings that would produce invalid code to be errors instead? IIRC one of the warnings looks like "This parameter type is not supported. Generated code won't handle marshaling this parameter." I think it would be confusing if this warning meant that the function pointer in the VTable would be null.

[jkoritzinsky]

Should we change all the warnings that would produce invalid code to be errors instead? IIRC one of the warnings looks like "This parameter type is not supported. Generated code won't handle marshaling this parameter." I think it would be confusing if this warning meant that the function pointer in the VTable would be null.

They already are errors. We have very few warnings generated by the generator nowadays.

[jkoritzinsky]

Failures are unrelated.