Draft: Add clang to packages (for Allen)

Allen needs to use libclang, see lhcb/Allen!806 (merged)

fragments/packages/LCG_101-x86_64-centos7-gcc11-opt.cmake

@@ -565,6 +565,7 @@ _add_lcg_entry("${LCG_releases_base}/MCGenerators/thep8i/2.0.0-cd57d/x86_64-cent
 _add_lcg_entry("${LCG_releases_base}/ROOT/6.24.06-3455f/x86_64-centos7-gcc11-opt")
 _add_lcg_entry("${LCG_releases_base}/DD4hep/01.18-e4540/x86_64-centos7-gcc11-opt")
 _add_lcg_entry("${LCG_releases_base}/GitCondDB/0.2.0-1c8c6/x86_64-centos7-gcc11-opt")
+_add_lcg_entry("${LCG_releases_base}/clang/12.0.0/x86_64-centos7")
 
 list(PREPEND ld_library_path "${LCG_releases_base}/catboost/0.26.1-86407/x86_64-centos7-gcc11-opt/catboost/libs/model_interface")
 _add_lbenv_workspace(${LHCB_PLATFORM})