Fix test regressions after LLVM 15 upgrade

[dan-zheng]

This issue tracks test regressions after upgrading to LLVM 15 in #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)

$ # 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.