Fix test regressions after LLVM 15 upgrade

[dan-zheng]

This issue tracks test regressions after upgrading to LLVM 15 in https://github.com/google-research/dex-lang/pull/1264.

Test regression types:

• Segfaults, both deterministic and nondeterministic
• Unstable numerical results
  • Different results between llvm-15 and main
  • Different results between Linux and macOS on llvm-15

Tests to fix

• [ ] examples/psd.dx
• [ ] examples/nn.dx
• [ ] examples/tutorial.dx
• [ ] examples/mcmc.dx
• [ ] makefile
  • [ ] examples/fluidsim.dx examples/schrodinger.dx: segfault due to :html command
  • [ ] examples/mcts.dx: nondeterministic segfault
  • [ ] tests/set-tests.dx: deterministic segfault from to_set
• [ ] tests/eval-tests.dx
• [ ] tests/sort-tests.dx
• [ ] tests/adt-tests.dx
• [ ] python/tests/dexjit_test.py:test_concat_ragged: segfault

Raw occurrences of FIXME(llvm-15)

```console $ # From https://github.com/google-research/dex-lang/tree/llvm-15 rg "FIXME\(llvm-15\)" examples/nn.dx 168:-- FIXME(llvm-15): Re-enable lines below. Currently crashes with segfault. examples/tutorial.dx 376:-- FIXME(llvm-15): Re-enable lines below. Currently crashes with segfault. 627:-- FIXME(llvm-15): Re-enable lines below. Currently crashes with segfault. examples/mcmc.dx 106:-- FIXME(llvm-15): Re-enable this. 119:-- FIXME(llvm-15): Re-enable this. examples/psd.dx 34:-- FIXME(llvm-15): Re-enable this. makefile 224:# fluidsim schrodinger # FIXME(llvm-15): segfault due to `:html` command 225:# mcts # FIXME(llvm-15): nondeterministic segfault 240:# set-tests # FIXME(llvm-15): segfault tests/eval-tests.dx 221:-- FIXME(llvm-15): Re-enable this. tests/sort-tests.dx 49:-- FIXME(llvm-15): Re-enable below. tests/adt-tests.dx 263:-- FIXME(llvm-15): Fix segfault on llvm-15 branch. 277:-- FIXME(llvm-15): Fix segfault on llvm-15 branch. 282:-- FIXME(llvm-15): Fix segfault on llvm-15 branch. ```