Revert "[llvm][cmake] Make `llvm_install_symlink` robust to absolute dirs."

https://lab.llvm.org/buildbot/#/builders/36/builds/16668 was the sort of
thing I saw before when this was part of D99484, and it makes some sense
now this would have something to do with it.

This reverts commit 58580e922a.

llvm/cmake/modules/LLVMInstallSymlink.cmake

@@ -6,8 +6,7 @@ include(GNUInstallDirs)
 
 function(install_symlink name target outdir)
   set(DESTDIR $ENV{DESTDIR})
-  GNUInstallDirs_get_absolute_install_dir(bindir "${outdir}" BINDIR)
-  set(bindir "${DESTDIR}${bindir}")
+  set(bindir "${DESTDIR}${CMAKE_INSTALL_PREFIX}/${outdir}")
 
   message(STATUS "Creating ${name}")