[llvm][lldb] use FindLibEdit.cmake everywhere

Currently, LLVM's LineEditor and LLDB both use libedit, but find them in different (inconsistent) ways.
This causes issues e.g. when you are using a locally installed version of libedit, which will not be used
by clang-query, but by lldb if picked up by FindLibEdit.cmake

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D124673