Update LLVM libraries to version 20

[sbc100]

LLVM version 20.1 was released on March 4th so its time for our semi-annual llvm library upgrade.

See #23294 for example.

[sbc100]

@aheejin do you want me to take this one, or do you want to take it on again?

[aheejin]

I'm happy to do that.

[sbc100]

I think this can be closed now? Or is there more to do?

[aheejin]

Closed by #24251, #24346 and #24357.