[cmake] Detect presence of wide-char libedit at build time

Summary:
Instead of hardcoding a list of platforms where libedit is known to have
wide char support we detect this in cmake. The main motivation for this
is attempting to improve compatibility with different versions of
libedit, as the interface of non-wide-char functions varies slightly
between versions.

Reviewers: krytarowski, uweigand, jankratochvil, timshen, beanz

Subscribers: mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D47625

llvm-svn: 334393