Add LLVM copyright notices from `polytracker-llvm`

[surovic]

We could consider adding the git commit hash of the version from which we forked, but I think this is otherwise fine. We might want to update our base README with copyright notices from forked/vendored LLVM code if we haven't already done so.

Originally posted by @ESultanik in https://github.com/trailofbits/polytracker/pull/6517#discussion_r1116006242