Use smaller keys in language service caches

[cartermp]

#5944 was closed after #6001 came in because we have a different way that source files are passed into the language service, eliminating a large amount of allocations. However, these caches:

https://github.com/Microsoft/visualfsharp/blob/4b7ea0805935112cfaad884dadb7b1af17cdf190/src/fsharp/service/service.fs#L2472

https://github.com/Microsoft/visualfsharp/blob/4b7ea0805935112cfaad884dadb7b1af17cdf190/src/fsharp/service/service.fs#L2455

https://github.com/Microsoft/visualfsharp/blob/4b7ea0805935112cfaad884dadb7b1af17cdf190/src/fsharp/service/service.fs#L2999

All still end up storing an entire source file as a part of a key. This keeps large strings around needlessly. Instead, we should use a hash code (or some hash-like representation of the source) that we use as a part of the key. This should help further relieve memory pressure for larger solutions.