[swift interop] Cannot call instance methods with at least one parameter when using `SwiftSelf<T>`

[jkurdek]

Our current interop implementation only allows SwiftSelf (representing the self parameter in Swift instance methods) to be specified as the first parameter in function signatures. This design choice leads to issues when interfacing with Swift instance methods that accept additional parameters.

Let's consider following example

@frozen public struct SampleStruct
{
    public var x1: Int
    public var x2: Int

    public func addWithExtraArgs(x3: Float, x4: Float) -> Float {
        return Float(x1 + x2) + x3 + x4
    }
}

addWithExtraArgs results in the following LLVM-IR:

define protected swiftcc float @"output.SampleStruct.addWithExtraArgs(x3: Swift.Float, x4: Swift.Float) -> Swift.Float"(float %0, float %1, i64 %2, i64 %3) ...

SampleStruct is expected to be lowered into two i64 which are passed as last parameters.
This would correspond to following csharp signature:

static extern void AddWithExtraArgs(float a, float b, SwiftSelf<SampleStruct> struct)

We should either allow SwiftSelf<T> argument to be specified as the last parameter or we should reorder the parameters in runtime.

\cc: @kotlarmilos, @jakobbotsch, @amanasifkhalid, @matouskozak

[dotnet-policy-service]

Tagging subscribers to 'os-ios': @vitek-karas, @kotlarmilos, @ivanpovazan, @steveisok, @akoeplinger
See info in area-owners.md if you want to be subscribed.

[dotnet-policy-service]

Tagging subscribers to 'os-tvos': @vitek-karas, @kotlarmilos, @ivanpovazan, @steveisok, @akoeplinger
See info in area-owners.md if you want to be subscribed.

[dotnet-policy-service]

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