[CMake] Get Clang directory from CLANG_INSTALL_PREFIX

If Clang was installed standalone (in particular, not in the LLVM tree),
use the CLANG_INSTALL_PREFIX variable to indicate where it is.

Actually, we can always use `CLANG_INSTALL_PREFIX`. It's defined by
`ClangConfig.cmake`. We should also need to take into account
`LLVM_LIBDIR_SUFFIX`.

Author: guitargeek

core/clingutils/CMakeLists.txt

@@ -85,7 +85,7 @@ if (builtin_clang)
   set(CLANG_RESOURCE_DIR_STEM ${CMAKE_BINARY_DIR}/interpreter/llvm-project/llvm/${CMAKE_CFG_INTDIR}/lib/clang)
   set(CLANG_RESOURCE_DIR_VERSION ${LLVM_VERSION_MAJOR})
 else ()
-  set(CLANG_RESOURCE_DIR_STEM ${LLVM_LIBRARY_DIR}/clang)
+  set(CLANG_RESOURCE_DIR_STEM ${CLANG_INSTALL_PREFIX}/lib${LLVM_LIBDIR_SUFFIX}/clang)
   # A user can define a clang version to use, otherwise find it (but will error if more than one version is present)
   if (NOT DEFINED CLANG_RESOURCE_DIR_VERSION)
     if (NOT EXISTS ${CLANG_RESOURCE_DIR_STEM})