Make LLVM lowering deterministic

[camio]

The inability to get reproducible compiler error output frustrates debugging.

[dabrahams]

The most likely source of nondeterminism here is the random order that things appear in hash containers from run to run (that's a Swift security feature). If you want to straighten that out there's probably a .sorted() that could be inserted somewhere.

[kyouko-taiga]

We can define the order in which Hylo IR functions will be transpiled to LLVM. That will likely make our builds much more deterministic.

Unfortunately we only have partial control over the order in which LLVM will manage it's internal representation, run its optimization passes, etc. That may be another cause of nondeterministic errors which we can't address without patching LLVM.