workflows: test against LLVM 18.1

[airwoodix]

Description

Add a matrix entry for LLVM 18.1 in the testing workflow.

Requires updating to KyleMayes/install-llvm-action v2.0.2+. Breaking API changes from v2.0.0 are not relevant here.

Related Issue

#482, #524 (comment)

How This Has Been Tested

• Partial local run using nektos/act to check that the install-llvm-action resolves the LLVM version correctly.
• cargo test -F llvm18-0 with LLVM 18.1.8 libraries.
• Fork pipeline: https://github.com/airwoodix/inkwell/actions/runs/10459452802

Checklist

• I have read the Contributing Guide

[TheDan64]

Changes look good to me but tests are not passing

[airwoodix]

I was a bit fast reading the changelog for v2.0.0 in install-llvm-action. The API changes are not breaking the use here. However, support for LLVM < 7.1 was dropped in KyleMayes/install-llvm-action#64. The limitation seems to come from older releases not being served in the llvm/llvm-project releases.

• 7734b55 removes LLVM 4-6 from the testing matrix and bumps to 7.1 for the 7 series. Would this be acceptable?
• f8f9729 reverts to using ubuntu-20.04 (still shipping with libtinfo5, unlike more recent releases) for LLVM 18.

[TheDan64]

Maybe we could run the old version for <7 and the new one for newer versions?

[airwoodix]

Done in a25308b.

uses apparently doesn't support (ternary) expressions, so the "Install LLVM and Clang" step is duplicated, with exclusive pre-conditions.

[TheDan64]

Thank you!