Ability to query for available toolchain versions

Hello - I would like to be able to programmatically discover the set of toolchains I could possibly pass in to llvm.toolchain( llvm_version = "" ). We have bots that keep our dependency versions up to date, and such an api would help them figure out what string to put there.

Thank you!

Start at the llvm.toolchain entry point mentioned in the request and trace how llvm_version values are accepted and resolved. Review the existing repository configuration and tests, if present, to define how an enumeration API would expose available versions; done means bots can discover valid values programmatically.

Written by the indexing model from the issue text.