Upgrade to LLVM 20

[zanieb]

LLVM 20.1.0 was released a couple days ago https://github.com/llvm/llvm-project/releases/tag/llvmorg-20.1.0

We may want to include the following, which are not merged but are impactful for CPython performance:

• [TailDuplicator] Do not restrict the computed gotos llvm/llvm-project#114990
• [BOLT] Support computed goto and allow map addrs inside functions llvm/llvm-project#120267

[zanieb]

There's a recommendation to merge and backport from a team member

I propose to merge llvm/llvm-project#114990 and backport it. This issue has already affected multiple projects, and Python is a widely used one. In fact, this PR just reverts the computed goto part of llvm/llvm-project#78582.

ref llvm/llvm-project#106846 (comment)

[indygreg]

I didn't realize the needed PRs aren't merged yet :/

I think I'm going to put out a vanilla LLVM 20 release then a variant with the unmerged patches.

[indygreg]

Actually, the patches don't look too bad. I'm pretty liberal in my tolerance for BOLT patches. And the LLVM codegen patch is pretty trivial. I'll probably just take them both and do 1 release.

[zanieb]

Whatever you're more comfortable with.

[indygreg]

I've got CI runs in progress on the ci-test branch. (I don't bother with PRs in this repo.)

There's a bit of Actions Runners backlog due to other work in indygreg/python-zstandard.

Hopefully I'll get clean builds in ~3 hours. And if I'm around the computer later I should be able to publish a draft release.