[CMake] Handle absolute paths for symlinks

[sebastian-ne]

Summary

The binary installation path can already be absolute. In that case,
we must not prepend CMAKE_INSTALL_PREFIX. extend_path is used in other
places to handle that, this patch adds it when creating symlinks like
clang++ or llvm-readelf.

[sebastian-ne]

This is a duplicate of D101070, so I’ll close it.

llvm/cmake/modules/LLVMInstallSymlink.cmake

 # We need to execute this script at installation time because the
 # DESTDIR environment variable may be unset at configuration time.
 # See PR8397.
 
 include(GNUInstallDirs)
 
+set(CMAKE_MODULE_PATH "${CMAKE_CURRENT_LIST_DIR}/../../../cmake/Modules" ${CMAKE_MODULE_PATH})
+include(ExtendPath)
+
 function(install_symlink name target outdir)
   set(DESTDIR $ENV{DESTDIR})
-  set(bindir "${DESTDIR}${CMAKE_INSTALL_PREFIX}/${outdir}")
+  extend_path(prefixed_outdir "${CMAKE_INSTALL_PREFIX}" "${outdir}")
+  set(bindir "${DESTDIR}${prefixed_outdir}")
 
   message(STATUS "Creating ${name}")
 
   execute_process(
     COMMAND "${CMAKE_COMMAND}" -E create_symlink "${target}" "${name}"
     WORKING_DIRECTORY "${bindir}" ERROR_VARIABLE has_err)
   if(CMAKE_HOST_WIN32 AND has_err)
     execute_process(
       COMMAND "${CMAKE_COMMAND}" -E copy "${target}" "${name}"
       WORKING_DIRECTORY "${bindir}")
   endif()
 
 endfunction()