Restore LLVM to the correct revision

PR #12407 was accidentally reverted by PR #12411. Restore the correct
version of LLVM that PR #12407 introduced.
auto merge of #12462 : kballard/rust/restore-llvm-from-12407, r=brson

PR #12407 was accidentally reverted by PR #12411. Restore the correct
version of LLVM that PR #12407 introduced.