[clingutils] build regression between 6.40 and 6.42 with builtin_llvm=OFF

### Check duplicate issues.

- [x] Checked for duplicates

### Description

Consider the following:

```
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 ${ROOT_LLVM_VERSION_REQUIRED_MAJOR}.1 REQUIRED|g' interpreter/CMakeLists.txt # Backport of a patch from master
mkdir build && cd build
cmake -Dbuiltin_llvm=OFF -Dminimal=ON ..
make -j$(nproc)
./bin/root
   ------------------------------------------------------------------
  | Welcome to ROOT 6.40.03                        https://root.cern |
```

runs and builds totally fine. In particular: `[ 55%] Built target LLVMRES` looks ok.

Instead in current master:

```
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 ..
make -j$(nproc)
```

fails with

```
[ 54%] Copying LLVM resource and header files
Error copying directory from "interpreter/llvm-project/llvm/./lib/clang/22/include" to "etc/cling/lib/clang/22/include".
make[2]: *** [core/clingutils/CMakeFiles/LLVMRES.dir/build.make:88: CMakeFiles/LLVMRES.stamp] Error 1
make[1]: *** [CMakeFiles/Makefile2:24796: core/clingutils/CMakeFiles/LLVMRES.dir/all] Error 2
make: *** [Makefile:156: all] Error 2
```

So in the upgrade from LLVM20 to LLVM22 I believe that something has affected the core/clingutils/LLVMRES target. I can get past this point in the build by commenting the failing command:

```
-        COMMAND ${CMAKE_COMMAND} -E copy_directory
-        ${CLANG_RESOURCE_DIR}
-        ${CMAKE_BINARY_DIR}/etc/cling/lib/clang/${CLANG_RESOURCE_DIR_VERSION}/include
+#        COMMAND ${CMAKE_COMMAND} -E copy_directory
+#       ${CLANG_RESOURCE_DIR}
+#        ${CMAKE_BINARY_DIR}/etc/cling/lib/clang/${CLANG_RESOURCE_DIR_VERSION}/include
```

but  a proper fix is needed because otherwise it obviously fails later with the message:

```
fatal error: module map file '/root_src/build/etc/cling/lib/clang/22/include/module.modulemap' not found
fatal error: module map file '/root_src/build/etc/cling/lib/clang/22/include/module.modulemap' not found
```

which I can then manually solve by calling:

`cp -p -r /root_src/build/lib/clang/22/include/ /root_src/build/etc/cling/lib/clang/22/`


### 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 ..
make -j$(nproc)
```

### ROOT version

```
ROOT v6.40.02
Built for linuxx8664gcc on Jun 10 2026, 10:37:47
From tags/v6-40-02@v6-40-02
With c++ (Ubuntu 13.3.0-6ubuntu2~24.04.1) 13.3.0 std201703
Binary directory: /opt/root/bin
```

### Installation method

Source

### Operating system

Ubuntu26 Docker

### Additional context

tagging @guitargeek and @vepadulano since they have experience with builtin_llvm=OFF

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