Better generic unmanaged structs handling ++ bidirectional F#/C# interop for 'unmanaged' constraint

[vzarytovskii]

Pretty much a continuation of @TIHan's work on on top of main (previous PR for reference: #6064).
RFC: https://github.com/fsharp/fslang-design/blob/main/RFCs/FS-1090-Generic-struct-type-whose-fields-are-all-unmanaged-types-is-unmanaged.md

TODO/FIXES:

• Treat type arguments with IsUnmanagedAttribute as if they have the unmanaged constraint.
• Emit IsUnmanagedAttribute for the unmanaged constraints on params.
• Emit UnmanagedType modreq on type arguments on types
  - This would be a binary breaking change, we should consider if we want to do it or not.
• An unmanaged constraint cannot be used together with not struct constraint (not the case right now).
• Fix struct tuples, and struct anonymous records - whenwe check isUnmanaged on their arguments, tuple/record hasn't been solved yet.

** TEST list ** (Tomas's braindump)

• F# side definition options:
  • Generic custom types
  • Generic struct records
  • Struct tuples
  • Struct DU's
  • Struct anonymous records
  • Deeply nested generic scenarios ( MyStruct<MyOtherStruct<StructTuple<int,byte>>> )
  • Any nested combination of the following (e.g. AnonRecord<StructTuple<CustomType<Record>>>)
• Test matrix dimensions for interop (missing anything?)
  • Method with unmanaged constraint defined in C#/F# (*2 test cases)
  • Data to be passed (a generic struct) defined in C#/F# (*2 test cases)
  • The T is either valid or invalid and should show error (*2 test cases)
  • The callsite is C#/F# (*2 test cases)
• Additional test pointers:
• If we do not put in a modreq in F# codegen for a constrained method,:
  • C# can still call it
  • C# can still typecheck it for in/correct usage
  • VB.NET is not capable of calling it incorrectly
    • Well, apparently, VB.NET does not support this feature and is not capable of calling into such methods.
  • For places where C# puts in a modreq, F# can call those methods (haven't checked deeply yet, might differ across virutal/non-vritual scenarios)

[vzarytovskii]

Emit UnmanagedType modreq on type arguments.

I am not sure whether we should emit it just because C# does it as well, since it will change the default behaviour we already have, it will also be not easy to test.

[T-Gro]

Before this PR, the unmanaged constraint defined in F# was erased for IL output, therefore any other .NET consumer could call into such defined types/methods.

With this PR, the modreq is being emitted.
Which F# and C# understands, but VB.NET does not:

dotnet/vblang#300
dotnet/vblang#604

Which means that public APIs that should also target VB.NET will need to be aware of this and provide alternatives.

[Happypig375]

voidptr support? fsharp/fslang-suggestions#1160

[T-Gro]

voidptr support? fsharp/fslang-suggestions#1160

I don't see why not, I can add it in a separate PR right after this one is merged in.