New LLVM Version Deprecation Policy

[TheDan64]

I've been thinking about how older LLVM versions add a lot of tech debt / baggage. Previously we had dropped support for the 3.x series because they were just extremely buggy and causing a lot of problems. This is currently our only precedent for removal. I've always wanted inkwell to support as many versions as possible, but a 14 major version range is pretty huge. Newer ubuntu versions are also dropping support for older LLVMs, which makes testing them harder as we have to keep older OS versions around too. It's time we take an official stance on deprecation / removal since LLVM versions are a constantly moving target.

What I'm proposing is that we drop support for any version older than 5 years. This means, 4, 5, 6 are slated for removal, effective the next release (0.6.0). LLVM 7.0.x technically fits this bill as well, but since 7.1 was released in 2019, it just barely gets grandfathered in. Whether or not it gets removed will depend on if 0.6.0 is released this year or next.

Please let us know if you have any questions or concerns about the new policy. Thanks!