Revert "Use `GNUInstallDirs` to support custom installation dirs. -- LLVM"

This reverts commit 6fd2db04 since it
broke GreenDragon LLDB-Incremental bot:

https://green.lab.llvm.org/green/job/lldb-cmake/37560/console



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>