[CppCodegen] Simple file I/O does not work

[jkotas]

Forked from #2147 (comment)

Repro:

var fs = new FileStream("bob.txt", FileMode.Append);

With CppCodeGen, it fails with:

C:\Users\Fabian\Documents\Development\nativetest\Program.cs(109752): error C2440: 'initializing': cannot convert from 'System_Private_CoreLib::System::Threading::NativeOverlapped *' to 'intptr_t' [C:\Users\Fabian\Documents\Development\nativetest\nativetest.csproj]
C:\Users\Fabian\Documents\Development\nativetest\Program.cs(109752): note: There is no context in which this conversion is possible

[jkotas]

This one should be easy one to fix. https://github.com/dotnet/corert/blob/master/src/ILCompiler.Compiler/src/CppCodeGen/ILToCppImporter.cs is the place to look to get it fixed.