[cmake] Remove usage of --src-root with llvm-config

[stephanlachnit]

This Pull request:

Changes or fixes:

Remove usage of --src-root, which is removed in LLVM16 (#13273). See also hahnjo#1. Only relevant for external LLVM and works without this anyway.

Checklist:

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

[phsft-bot]

Starting build on ROOT-debian10-i386/soversion, ROOT-performance-centos8-multicore/cxx17, ROOT-ubuntu2204/nortcxxmod, ROOT-ubuntu2004/python3, mac11/noimt, mac12arm/cxx20, windows10/default
How to customize builds

[hahnjo]

To elaborate based on the motivation to remove --src-root from llvm-config (llvm/llvm-project@c061892): The returned path might sometimes work if LLVM was built and installed on the same machine. The likelier setup is wanting to use distributed binaries of LLVM, in which case --src-root doesn't exist anyway.

[vgvassilev]

Makes sense to me.

[github-actions]

Test Results

         9 files           9 suites   2d 5h 38m 32s ⏱️
  2 476 tests   2 474 ✔️ 0 💤 2 ❌
21 183 runs  21 181 ✔️ 0 💤 2 ❌

For more details on these failures, see this check.

Results for commit 23a56fb.