Tweak SimpleFastHash

This change adds a SimpleFastHash64 variant of SimpleFastHash which allows call sites to specify a starting value and get a 64 bit hash in return. This allows a hash to be "resumed" with more data.

A later patch needs this to be able to hash a sequence of module-relative values one at a time, rather than just a region a memory.

Reviewed By: morehouse

Differential Revision: https://reviews.llvm.org/D94510