Resolves https://devdiv.visualstudio.com/DevDiv/_workitems/edit/2208409

Issue was caused by #74544

Prior to my change, absolute URIs would ignore case when comparing UNC URIs (default impl of Uri.GetHashCode. However once I manually started comparing AbsolutePaths, case was never ignored (due to the hashcode being different for upper vs lower case strings).

This showed up as an issue in C# DevKit because files in the workspace get added with an upper case drive letter (by devkit project system). But VSCode sends LSP requests always with a lower case drive letter. We'd end up always ignoring the diagnostics requests because we thought the file was not tracked by LSP.

This did not show up as an issue in the C# extension because we populate the workspace there with lower case drive letters (what vscode gives us).

The change here is to use an ignoring case hashcode for absolute path. This will potentially cause collisions if a path exists with both cases, but that is generally rare. We will fallback to the URI implementation of comparison in Equals, which appropriately handles case depending on the kind of URI.