PR Summary

Related #6874. The PR is coming from a code review to cleanup psl. Here we clean up the side branch of the code that will allow later to clean up a branch which uses psl.

• IO.FileInfo does not make system calls in constructor. So we can create the object and then use the required attributes without direct call IO.FileInfo.GetAttributes() ( SafeGetFileAttributes() ). This allow us to exclude some p/invoke calls in our code in later cleanups. Also we get unified code for both Windows and Unix.
• Remove SafeGetFileAttributes() and WinSafeGetFileAttributes(). Currently .Net Core support file attributes on all platforms in fastest way and we can remove our workaround. We get a regression in rare case (for files like pagefile.sys). Fix is ready in CoreFX, we get it in 2.1.1. I suggest ignore the regression because this is a very-very rare situation (Get-ChildItem c:\pagefile.sys -Hidden). The .Net Core team was not even able to create an artificial test for such files and uses a real pagefile.sys file for the test. Also the enumeration is still working (dir c:\ -hidden).
• Re-add test which we lost in Make CodeCoverage tests work with CodeCov.io and related fixes #4050. The test is pending because of the regression.

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