[lldb] Fix editline unicode on Linux

Based on:
  [lldb-dev] proposed change to remove conditional WCHAR support in libedit wrapper
  https://lists.llvm.org/pipermail/lldb-dev/2021-July/016961.html

There is already setlocale in lldb/source/Core/IOHandlerCursesGUI.cpp
but that does not apply for Editline GUI editing.

Unaware how to make automated test for this, it requires pty.

Reviewed By: teemperor

Differential Revision: https://reviews.llvm.org/D105779