Install LLVM via pinned tarball

[kerberjg]

Features

• Fixes Ubuntu 24.04 LLVM installation fails #169
• Allows LLVM installations to be pinned by semver* (see workaround below)
• Still respects PREFER_LLVM ! (default stored in config/globals.json)
• LLVM installation is confined to workspace, doesn't affect OS* (see workaround below)
• Installation is fully cache-ready (following Implements setup-workspace GitHub Action + caching #172)
• Moves Wayland+MESA package installation to a separate JSON config

Build matrix behavior:

• Builds only with LLVM 18 by default
• Always builds with LLVM 14+18 on ubuntu-legacy
• Builds other LLVM versions when the config for that version or the relevant OS have changed

*workaround: in the following cases there are no compatible LLVM tarballs available, so LLVM is installed from the system package manager instead:

• Ubuntu 20.04 w/ LLVM >= 18
• Ubuntu 22.04 x86_64 w/ LLVM 14
• macOS x86_64 w/ LLVM 16, 17, 18
• macOS arm64 w/ LLVM 16

---

Build matrix

v OS - LLVM >	14	15	16	17	18	19	20	21	22
Ubuntu (legacy, 20 - x86_64 only)	✅	✅	✅	✅	✅	✅	✅	✅	✅
Ubuntu (22, 24 - x86_64, aarch64)	✅	✅	✅	✅	✅	✅	✅	✅	✅
Fedora (41,42,43 - x86_64 only)	-	-	-	-	✅	✅	✅	✅	✅
macOS (14,15) - arm64	✅	✅	✅	✅	✅	✅	✅	✅	✅

^ see reviewer notes at the bottom

Windows has been excluded because it doesn't use LLVM, and is thus unaffected

---

Takeaways:

• Fedora seems to be consistently 30% faster than Ubuntu
• Fedora 41 is past EOL (should we still support it?)
• macOS standa out because they don't try to compile anything with the toolchain (AFAIK)
• macOS doesn't check against latest OS version
• macOS doesn't check against x86_64 arch
• RHEL is mentioned in some support arrays but not explicitly tested against
• Often jobs fail due to:
  • Download timeout (we should have a timeout/retry mechanism for this)
  • GHA Runner running out of disk space (shouldn't be happening, but we could still account for this)

---

Requires #170 to land first - refactor before merging.

TODO:

• Implement per-arch download of artifacts
  • Implemented per-OS artifacts #170
  • Implemented per OS-version artifacts #175
• Implement in-workspace isolated install
• Ubuntu 20.04 (legacy) workaround (see below)
• Write separate configs for major LLVM versions
• Compatibility with PREFER_LLVM env var
• Check macOS compatibility
• Refine build matrix (too large now)
• Cleanup

[kerberjg]

Currently features a workaround for ubuntu-legacy (20.04) - in absence of Ubuntu 20 (older glibc) -specialized builds of LLVM (see meta-flutter/llvm-toolchains#1), it will install LLVM from its official APT repository. This might prevent proper version pinning, but hasn't been a problem when testing (also - it's temporary)

[jwinarske]

Drop Fedora 41; EOL

[jwinarske]

Also on a change this large don’t skip anything; ensure everything is green.

[kerberjg]

@jwinarske Would it work if I drop Fedora 41 in #173 ? Since that one adds 44 support (and is otherwise ready to go)

[kerberjg]

@jwinarske CI green on everything