Consolidate file handle usage in Editline.cpp

Summary:
To implement wide character reading, editline was mixing FILE*-based access with
a Connection-based one (plus it did some selects on the raw FD), which is very
fragile. Here, I replace it with one which uses only a Connection-based reads.
The code is somewhat longer as I had to read characters one by one to detect the
end of the multibyte sequence.

I've verified that international characters still work in lldb command line on
OSX.

Reviewers: clayborg, zturner

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D28356

llvm-svn: 291220