LLVM CI: build LLVM from source

[HertzDevil]

The official Apt repositories only serve the 3 most recent major versions, while the volunteer-built packages are incomplete and of inconsistent quality, so the best solution is to build LLVM ourselves.

This also reduces the total size for the 6 LLVM caches from 8 GB to around 2.4 GB. That's more than half the limit for a GitHub repository.

lld is needed for #13193; technically speaking, we could build it straight from the very repository we're cloning here, but we aren't really testing any version-specific lld behavior, so this patch grabs it from Apt.

[oprypin]

This also reduces the total size for the 6 LLVM caches

Hmm actually it looks like there wasn't even any reason to have those caches before because it just moves the download+extract step from one place on GitHub to another. Now of course they'll be necessary. Looks good