[clang] do not search again for LLVM if already found by parent project

[ferdymercury]

This Pull request:

Changes or fixes:

In ROOT, for builtin_llvm=OFF we have the sequence

find_package(LLVM) # searchs for system package, which sets DYLIB to ON
set(LLVM_LINK_LLVM_DYLIB OFF) # override DYLIB to OFF, we want static
add_subdirectory(clang) # this will call find_package LLVM again

See #13420

Since LLVM22 though, this fails:
Even if we call for a second time set(LLVM_LINK_LLVM_DYLIB OFF) after the add_subdirectory line, it's not enough, because that variable was ON all throughout the child clang/CMakeLists.txt and has already configured some stuff with the wrong flag.

Avoid the issue by preventing the double-search of LLVM and thus the double overwrite.

Fixes #18387
Fixes #23155

Upstream PR: llvm/llvm-project#218965

Reproducer

docker run -it registry.cern.ch/root-ci/ubuntu2604:buildready
apt update
apt install llvm-20 libedit-dev libpolly-20-dev
git clone https://github.com/root-project/root root_src && cd root_src
git checkout v6-40-00-patches
sed -i 's|LLVM REQUIRED|LLVM ${ROOT_LLVM_VERSION_REQUIRED_MAJOR}.1 REQUIRED|g' interpreter/CMakeLists.txt # Backport of a patch from master
mkdir build && cd build
cmake -Dbuiltin_llvm=OFF -Dminimal=ON ..
grep -r LLVM.so *

Checklist:

• tested changes locally
• updated the docs (if necessary)

[github-actions]

Test Results

    23 files      23 suites   3d 13h 49m 20s ⏱️
 3 856 tests  3 856 ✅ 0 💤 0 ❌
78 596 runs  78 596 ✅ 0 💤 0 ❌

Results for commit 3f21c9a.

♻️ This comment has been updated with latest results.

[guitargeek]

Sorry, I have to remove the review request because I have to decided it's just easier to patch my system Clang build rather than arguing for patches that make builtin_llvm=OFF and builtin_clang=OFF work (pick your battles 🙂 ). There is strong resistance by @hahnjo (see e.g. #18387 (comment)), the code owner who you need to convince. I'm in favor of this patch, but it's not my area.

[hahnjo]

Yes, I don't see the point of adding more downstream patches for this case. I've said it before, the problem should be fixed "properly" by making it possible to link against a provided libLLVM.so. We know how to do that: #20903

[ferdymercury]

We know how to do that: #20903

Yep, I tried that PR as patch on my local build last week, but that does not help since it seems it moves the problem somewhere else. When applying that patch and building, I got:

[ 70%] Built target rootcling
[ 70%] Generating G__Net.cxx, ../../lib/Net.pcm
: CommandLine Error: Option 'lto-embed-bitcode' registered more than once!
LLVM ERROR: inconsistency in registered CommandLine options
Subprocess aborted
make[2]: *** [net/net/CMakeFiles/G__Net.dir/build.make:133: net/net/G__Net.cxx] Error 1
make[1]: *** [CMakeFiles/Makefile2:27235: net/net/CMakeFiles/G__Net.dir/all] Error 2
make: *** [Makefile:156: all] Error 2

Reproducer:

docker run -it registry.cern.ch/root-ci/ubuntu2604:buildready
apt update
apt install llvm-22 libpolly-22-dev
git clone https://github.com/root-project/root root_src && cd root_src
mkdir build && cd build
cmake -Dbuiltin_llvm=OFF -Dminimal=ON ..
wget https://github.com/root-project/root/pull/20903.patch?full_index=1
patch -p1 < 20903.diff
make -j($nproc)