Add a CI build for verifying PRs

[dpaoliello]

• Adds a new GitHub Action that is triggered for PRs.
• Verifies on Linux (GCC), MacOS (GCC) and Windows (MSVC).
• Linux and MacOS use a prebuilt LLVM, Windows builds its own (as llvm-dev is not available for Windows).
• Added caching for the Windows build (reduces build time from ~2hrs to 20mins).
• Fixed test failure (caused by truncating pointers to char and then sign-extending back to int).

Sample successful run: https://github.com/JuliaHubOSS/llvm-cbe/actions/runs/5590403992

Fixes #165