Remove use of string.GetHashCode(StringComparison) (#47883)

Remove use of string.GetHashCode(StringComparison) (#47883)

All of our call sites to it pass in a const StringComparison; GetHashCode will then turn around and decide which property on StringComparer to use, and then call GetHashCode on the relevant instance... we may as well just pick the instance directly, which not only saves the lookup and enables devirtualization, it also enables better trimming, as string.GetHashCode(StringComparison) ends up rooting all of the StringComparer properties in case you pass in the appropriate StringComparison.