Use MemoryMappedFile API instead of p/invoke win32 calls (#7944)
Use MemoryMappedFile API instead of p/invoke win32 calls (#7944)

* Remove win32 mmf calls. Use .NET MemoryMappedFile API instead

* Use DangerousGetHandle as it's an implementation detail

* minor cleanup

dsyme/fsharp,cloudRoutine/FSharp.Compiler.Service,cloudRoutine/FSharp.Compiler.Service,dsyme/fsharp,dsyme/fsharp,dsyme/fsharp,dsyme/fsharp,dsyme/fsharp,cloudRoutine/FSharp.Compiler.Service,dsyme/fsharp