Some minor inconsistencies in `service.fs` cache keying code.

Just a nitpick. Code for keying caches in `service.fs` caches could use some clean-up.

Right now what we have:

Probably unused
https://github.com/dotnet/fsharp/blob/93c135e032490dcdab0e9894256fe01ff42208cb/src/Compiler/Service/service.fs#L172

Alias
https://github.com/dotnet/fsharp/blob/93c135e032490dcdab0e9894256fe01ff42208cb/src/Compiler/Service/service.fs#L167

just `int64`
https://github.com/dotnet/fsharp/blob/93c135e032490dcdab0e9894256fe01ff42208cb/src/Compiler/Service/service.fs#L74

sometimes we pass fileVersion
https://github.com/dotnet/fsharp/blob/93c135e032490dcdab0e9894256fe01ff42208cb/src/Compiler/Service/service.fs#L730

sometimes we compute it
https://github.com/dotnet/fsharp/blob/93c135e032490dcdab0e9894256fe01ff42208cb/src/Compiler/Service/service.fs#L502

Here passed in `_fileversion` is ignored and hash is computed again instead.
https://github.com/dotnet/fsharp/blob/93c135e032490dcdab0e9894256fe01ff42208cb/src/Compiler/Service/service.fs#L469-L473

Also `int64` seems excessive, as we always convert from `int` anyway.