[lldb/CMake] Fix variable naming in FindLibEdit

The current FOUND_VAR for FindLibEdit is libedit_FOUND but wasn't set by
find_package_handle_standard_args. However this isn't valid for the
package name.

  The argument for FOUND_VAR is "libedit_FOUND", but only "LibEdit_FOUND" and
  "LIBEDIT_FOUND" are valid names.

This fixes all the variables set by FindLibEdit to match the desired
naming scheme.