Remove global state for typechecking patterns

[ncik-roberts]

This is a follow-on cleanup from #12117, inspired by @garrigue's comment about global state. @lpw25 and @goldfirere reviewed a version of this patch that was merged to Jane Street internally.

This patch replaces global variables managed by type_pat with a (mutable) state value that's explicitly passed around. There are a few benefits apart from the obvious "global mutable state is hard to reason about":

• the checking of or-patterns is a little more straightforward
• this makes it more obviously correct to remove the unnecessary module_patterns_restriction argument from partial_pred, exposed in the mli. (And, I do remove it in this PR.) These two things don't interact in an interesting way (module patterns, exhaustivity checking), and the explicit scoping of the type_pat_state value makes it obvious that we can just ignore module patterns here.

Implementation

The previous discipline was something like:

1. Call reset_pattern to reset the global state
2. Call type_pat (which writes into the global state)
3. Read out the results from the pieces of global state (the global variables pattern_variables, module_variables, and pattern_force)

The discipline is now:

1. Call create_type_pat_state to create tps, a fresh copy of pattern-checking state
2. Call type_pat on tps (which writes into its fields)
3. Read out the results from the fields of tps

I add a test to ensure that pattern_forces aren't dropped when checking or-patterns, as a draft of this PR made this mistake.

[gasche]

@t6s would you maybe be able to review this?

[goldfirere]

The Jane Street patch is at ocaml-flambda/flambda-backend#1281 if you want to see the conversations that happened during initial review.

[t6s]

The changes make the code simpler and better documented. Looks good to me.

As a side-product of my reviewing, I have done rebasing the branch to the current trunk,
with some comments added:
ncik-roberts#1

[t6s]

And, sorry for delaying my review so much.

[gasche]

Approving on behalf of @t6s. This is good to merge once rebased.

[ncik-roberts]

Thanks @t6s for rebasing. I've updated this PR with the result of that.

[gasche]

Merged, thanks @ncik-roberts for the nice patch and @t6s for the review.

Note: in the future (I'm noticing this as I just merged and I don't think it is important enough for this particular PR to request a change after the fact), I think it would be perfectly legitimate to credit people who reviewed on the internal flambda-backend repository (here Antal and Leo) as long as you consider their review was a substantial contribution (as far as reviews are concerned). (This is common practice in the Linux community where commit message tags are used to track review or testing work over the complex lifetime of each patchset.)

[ncik-roberts]

Ah, yes, that was a mistake—I thought to myself when I first opened this PR that I wanted to credit them as reviewers. Anyway, I agree it doesn't seem worth it to fix this omission in this case, but thanks for mentioning this.