There's a merged pull request on the void-packages repository

crystal: upgrade to llvm9
https://github.com/void-linux/void-packages/pull/19221

Description:
LLVM9 support was not announced (due to having non-breaking changes). Arch has been building with LLVM9 since 0.31.1-4, [source](https://git.archlinux.org/svntogit/community.git/commit/trunk?h=packages/crystal&id=33c30ed89d3a614862162d3e61085cc54c0e87b8).

Tested locally on x86_64 (both `crystal` and `shards`).

cc @gcat432