[EditLine] Fix RecallHistory to make it go in the right direction.

The naming used by editline for the history operations is counter
intuitive to how it's used in lldb for the REPL.

 - The H_PREV operation returns the previous element in the history,
   which is newer than the current one.
 - The H_NEXT operation returns the next element in the history, which
   is older than the current one.

This exposed itself as a bug in the REPL where the behavior of up- and
down-arrow was inverted. This wasn't immediately obvious because of how
we save the current "live" entry.

This patch fixes the bug and introduces and enum to wrap the editline
operations that match the semantics of lldb.

Differential revision: https://reviews.llvm.org/D70932