Add support for C# 9.0 function pointers

[petarpetrovt]

Currently DefaultDocumentation skips methods if they have function pointer argument.

Example:

/// ...docs
public unsafe static void SortDescending<T>(ref T p0, in int length, delegate* managed<T, T, int> comparisonPointer)
{
}

[Doraku]

Hopefully as soon as this is added into ICSharpCode.Decompiler it will pop up naturally in DefaultDocumentation, I will keep an eye on this.

[Doraku]

Fields,properties and parameters should work correctly, but it seems there is a bug in the xml documentation generated for method:

public unsafe void DummyMethodFunctionPointer(int _, delegate*<void> function) => function();

generate in xml (note the missing function pointer parameter...)

<member name="M:Dummy.DummyClass.DummyMethodFunctionPointer(System.Int32,)">

Because of this the method can't find its documentation. Since C#9 is still in preview I will wait for the official release to see if this bug is still there.

[Doraku]

waiting for dotnet/roslyn#46674 to be fixed