Replace HashHelpers.Combine with HashCode.Combine

[jcdickinson]

System.HashCode was recently added in order to make high-quality fast hashing easy. We should replace usage of HashHelpers.Combine, an internal low-quality hash combiner with HashCode.Combine. It should lead to better distribution in dictionaries and the hashing should be nearly as fast.

See: https://github.com/dotnet/coreclr/issues/15508

[danmoseley]

@jcdickinson interested in finishing this off?

[jcdickinson]

@danmosemsft there was a problem with completing this at the time. I had completed the majority of the work but was forced to gradually revert them. HashCode wasn't available in netfx and #if seemed like the wrong route, I decided to hold off until it shipped in netfx (as I assumed the vast majority of the alignment would be eventually reverted). Has HashCode shipped in netfx yet?

[Joe4evr]

Has HashCode shipped in netfx yet?

Doesn't look like it.