[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`. To make sure that this works, `find_package(Clang)`
has to be called in `core/clingutils/CMakeLists.txt`.

Author: guitargeek

core/clingutils/CMakeLists.txt

@@ -85,7 +85,14 @@ if (builtin_clang)
   set(CLANG_RESOURCE_DIR_STEM ${CMAKE_BINARY_DIR}/interpreter/llvm-project/llvm/${CMAKE_CFG_INTDIR}/lib${LLVM_LIBDIR_SUFFIX}/clang)
   set(CLANG_RESOURCE_DIR_VERSION ${LLVM_VERSION_MAJOR})
 else ()
-  set(CLANG_RESOURCE_DIR_STEM ${LLVM_LIBRARY_DIR}/clang)
+
+  # We also do find_package(Clang) in the interpreter directory, but since this
+  # CMakeLists file is not part of that dir we need to find Clang also here.
+  # Otherwise, the CLANG_INSTALL_PREFIX variable won't be set.
+  find_package(Clang REQUIRED CONFIG)
+
+  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})