GetClangResourceDir: Fix downstream projects that bundle llvm source

[tstellar]

Summary

A project that bundles the llvm source code may have their own
PACKAGE_VERSION variable, so only use this to compute the
CLANG_RESOURCE_DIR if CLANG_VERSION_MAJOR is undefined.

[sebastian-ne]

Thank you!

cmake/Modules/GetClangResourceDir.cmake

 # get clang resource directory
 #
 # usage:
 #  get_clang_resource_dir(out_var [PREFIX prefix] [SUBDIR subdirectory])
 #
 # user can use `PREFIX` to prepend some path to it or use `SUBDIR` to
 # get subdirectory under clang resource dir
 
 function(get_clang_resource_dir out_var)
   cmake_parse_arguments(ARG "" "PREFIX;SUBDIR" "" ${ARGN})
 
   if(DEFINED CLANG_RESOURCE_DIR AND NOT CLANG_RESOURCE_DIR STREQUAL "")
     set(ret_dir bin/${CLANG_RESOURCE_DIR})
   else()
+    if (NOT CLANG_VERSION_MAJOR)
       string(REGEX MATCH "^[0-9]+" CLANG_VERSION_MAJOR ${PACKAGE_VERSION})
+    endif()
     set(ret_dir lib${LLVM_LIBDIR_SUFFIX}/clang/${CLANG_VERSION_MAJOR})
   endif()
 
   if(ARG_PREFIX)
     set(ret_dir ${ARG_PREFIX}/${ret_dir})
   endif()
   if(ARG_SUBDIR)
     set(ret_dir ${ret_dir}/${ARG_SUBDIR})
   endif()
 
   set(${out_var} ${ret_dir} PARENT_SCOPE)
 endfunction()