[CMake] Support standalone builds of Clang

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

This is the same as 02b0dc1, but only changing the behavior when
`CLANG_INSTALL_PREFIX` is set without any other changes. This should
be a much less risky change (see also the revert in 6179736).

Author: guitargeek

core/clingutils/CMakeLists.txt

@@ -85,7 +85,13 @@ 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)
+  # If Clang was installed standalone (in particular, not in the LLVM tree),
+  # use the CLANG_INSTALL_PREFIX variable to indicate where it is.
+  if(DEFINED CLANG_INSTALL_PREFIX)
+    set(CLANG_RESOURCE_DIR_STEM ${CLANG_INSTALL_PREFIX}/lib/clang)
+  else()
+    set(CLANG_RESOURCE_DIR_STEM ${LLVM_LIBRARY_DIR}/clang)
+  endif()
   # 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})