Delete unnecessary MarshalImpl abstraction

[jkotas]

MarshalImpl is an extra layer without clear purpose. Also made MarshalAdapter to be included in CoreCLR/Mono builds only.

[MichalStrehovsky]

This broke Project N build because MCGConsole (the CoreCLR version of MCG) is misusing the System.Private.ILToolchain contract (and the associated WellKnownTypes class) to access the MarshalAdapter class (even though the class is definitely not part of the Project N ILToolchain contract).

The build fails generating the facade with MarshalAdapter removed.

We could fix this by either making MCGConsole stop using WellKnownTypes for this (not sure what's the next best mechanism though), or putting the useless type back into S.P.Interop. Longer term, MCGConsole should probably stop doing this though.

@luqunl Do you have any suggestions?

[jkotas]

I think just put the useless type back to S.P.Interop.

[luqunl]

In short, We need MarshalAdapter class to help us to solve Marshal API problem for MCG on CoreCLR.

Currently, there are 3 kinds APIs in interop area:

• Marshal API: public API, used by customer and exposed in corelib or System.Private.Interop.
• McgMarshal API: internal use only API, MCG generated code will call McgMarshal API
• MarshalAdapter API: Use by McgOnCoreCLR which will call McgMarshal API first and if McgMarshal fails, calls Marshal API

Why We added MarshalAdatper API:
Suppose Customer calls some Marshal APIs directly, These Marshal API would call directly into runtime(coreclr.dll) and may fail if these API related to COM or even Delegate, since runtime(coreclr.dll) doesn't know MCG generated data/System.Private.Interop. So what We need is to replace these Marshal APIs into another APIs(such as MarshalAdapter) which will know MCG generated data. The reason why We add a new APIs instead of using McgMarshal API is because of delegate---MCG on CoreCLR Analyzers doesn't analyze call graph, so it doesn't know which delegate type passed on Marshal.GetFunctionPointerForDelegate unless the delegate is marked with UnmanagedFunctionPointerAttribute(it means MCG on CoreCLR won't able to collect all of these delegate type and generate marshaling code). so for Marshal.GetFunctionPointerForDelegate API, We need to check MCG generated data first(if Delegate contains COM interface arguments), if it fails, then call directly into runtime(coreclr.dll)