Follow-up on non-blocking perf microimprovements for FileSystemEntry and FileStatus

[carlossanlop]

In this PR: #52235 there were a couple of non-urgent issues that we can address separately from that PR:

• 1. Fix: FileSystemEntry.Attributes property is not correct on Unix #52235 (comment) Use StartsWith instead of a manual check of length+first char, when checking if a filename is hidden because it starts with a dot. This method is only reached when the user explicitly checks the hidden value of a file/folder, but not on the most common enumeration scenarios.
• 2. Fix: FileSystemEntry.Attributes property is not correct on Unix #52235 (comment) When verifying the value of the read-only flag of a file/folder, we always check the values of GetEUid and GetEGid, which are p/invokes. The cases where these are accessed do not have a direct impact in the most common enumeration scenarios, but it would be nice to cache them and make sure their values get refreshed if the user requests doing so, or the file cache is uninitialized.
• 3. Fix: FileSystemEntry.Attributes property is not correct on Unix #52235 (comment) In FileSystemEntry.Initialize, make sure to put the check for symlink in an else if instead of an if, since having either unknown or symlink are mutually exclusive cases.
• 4. Fix: FileSystemEntry.Attributes property is not correct on Unix #52235 (comment) Consider reusing IsNameHidden when checking for the dot in FileSystemEntry.Initialize. A ROS<char> needs to be created from the byte*, and we need to check if aggressive inlining could help with any potential perf hit from calling two new methods.
• 5. Fix: FileSystemEntry.Attributes property is not correct on Unix #52235 (comment) Use the existing property to check if it's a symlink.
• 6. Fix: FileSystemEntry.Attributes property is not correct on Unix #52235 (comment) No need to check again for symlink when looking for attributes to skip.
• 7. HasNameHidden needs to be checked before EnsureCachesInitialized, inside IsHidden;.
• 8. Need to add a new OSX only file to check for UF_HIDDEN, since this does not apply to Unix.
• 9. Add perf benchmarks that consume AttributesToSkip.

Edit. I followed up in the same PR.

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Issue Details

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In this PR: #52235 there were a couple of non-urgent issues that we can address separately from that PR:

• 1. Use StartsWith instead of a manual check of length+first char, when checking if a filename is hidden because it starts with a dot. This method is only reached when the user explicitly checks the hidden value of a file/folder, but not on the most common enumeration scenarios.
• 2. When verifying the value of the read-only flag of a file/folder, we always check the values of GetEUid and GetEGid, which are p/invokes. The cases where these are accessed do not have a direct impact in the most common enumeration scenarios, but it would be nice to cache them and make sure their values get refreshed if the user requests doing so, or the file cache is uninitialized.

Author:	carlossanlop
Assignees:	carlossanlop
Labels:	area-System.IO
Milestone:	6.0.0