[SizzleUnrlsd]

Summary

The analyzer has not compiled with LLVM 19 since 33fe390, yet CMake still accepts LLVM 19 and scripts/build.sh still looks for it. This PR raises CMake's minimum LLVM version to 20, makes scripts/build.sh look for Homebrew llvm@20 first, and updates the documents that name an LLVM version.

What breaks

• 33fe390 (feat(resource-lifetime): MissingRelease from the ownership engine) reads the callee's captures attribute in src/analysis/ownership/ResourceFactCollector.cpp (lines 955 to 966) through llvm::CaptureInfo, llvm::CaptureComponents, llvm::capturesNothing and llvm::Argument::getAttributes(). All four first appear in LLVM 20, along with the captures attribute itself.
• Checked with g++ -fsyntax-only against apt.llvm.org's packages, that file has 9 errors with LLVM 19.1.7, all in those lines, and none with LLVM 20.1.8.
• CMakeLists.txt sets LLVM_MIN_REQUIRED_VERSION to 19, so a build with LLVM 19 passes configuration and fails during compilation. Every CI job installs LLVM 20, so none of them saw it.
• On macOS, scripts/build.sh takes the first Homebrew formula it finds among llvm, llvm@20, llvm@19 and llvm@18. Homebrew's llvm formula is LLVM 23.1, and the analyzer does not build with LLVM 21 (Build and test with LLVM 21 (Ubuntu 26.04 runners) #259), so with llvm and llvm@20 both installed, the script picks llvm.

Change

• CMakeLists.txt: the version check requires LLVM 20, in new and existing build directories alike. With LLVM 19, configuration now stops with LLVM version 19.1.7 found, but version 20 or higher is required. The minimum goes straight to check_llvm_version, instead of through the LLVM_MIN_REQUIRED_VERSION cache variable: set(CACHE) keeps a value already in the cache, so a build directory configured before this change would have kept 19. Nothing in the repository set that variable; a command line that still passes it gets CMake's warning about unused variables.
• scripts/build.sh: on macOS, the script looks for llvm@20, then llvm. It no longer looks for llvm@19 or llvm@18, which CMake would reject.
• CONTRIBUTING.md lists LLVM/Clang 20 instead of >= 19 (20 recommended). The README's build section states that the analyzer builds with LLVM and Clang 20, and how to install them on macOS and Ubuntu.
• CHANGELOG.md: an entry for this change, and one for fix(action): diagnose unsupported Ubuntu LLVM versions #257, which made the GitHub Action accept LLVM 20 only and added no entry.

Not in this PR

• CMake sets no maximum version: it still accepts LLVM 21 and later, which fail during compilation. Build and test with LLVM 21 (Ubuntu 26.04 runners) #259 tracks LLVM 21 support.
• On Linux, scripts/build.sh picks the first /usr/lib/llvm-* directory in alphabetical order, so with the development files of LLVM 18 or 19 next to those of LLVM 20, it picks the older version. CMake now rejects it at configuration, with the error above.

Verification

CMake, configured on macOS:

• With a stub LLVM package that reports version 19.1.7, configuration passed the version check before this change (Checking for LLVM >= 19..., then Found LLVM 19.1.7). It now stops with LLVM version 19.1.7 found, but version 20 or higher is required.
• In a build directory first configured from main with that stub package, reconfiguring with this PR's first version of CMakeLists.txt (aa648dc) still printed Checking for LLVM >= 19... and accepted it, because the cache kept 19. Reconfiguring with the current version stops with the error above.
• With Homebrew's LLVM 18.1.8, it stops with the same error, for 18.1.8.
• With Homebrew's LLVM 20.1.2 and the options CI passes, configuration and generation complete. Adding -DLLVM_MIN_REQUIRED_VERSION=19 only produces CMake's warning about unused variables.

scripts/build.sh --configure-only, run with stub brew and cmake commands, passes CMake the directories of these formulas:

Homebrew formulas installed	Before	After
llvm and llvm@20	llvm	llvm@20
llvm only	llvm	llvm
llvm@19 only	llvm@19	none: the script stops with LLVM_DIR not found
llvm@18 and llvm@20	llvm@20	llvm@20

CI does not run scripts/build.sh, and every job builds with LLVM 20, so this PR's own run checks that the new minimum accepts it.