D106035: Remove conditional compilation for WCHAR support in libedit

This change moves to using narrow character types and libedit APIs in
Editline, because those are the same types that the rest of LLVM/LLDB
uses, and it's generally considered better practice to use UTF-8
encoded in char than it is to use wider characters. However, for
character input, the change leaves in using a wchar to enable input of
multi-byte characters.

Differential Revision: https://reviews.llvm.org/D106035