Disable Editline on Windows.

Editline does not work correctly on Windows.  This goes back at
least to r208369, and as a result r210105 was submitted to disable
libedit at runtime on Windows.

More recently, r222163 was submitted which re-writes editline
entirely, but makes the situation even worse on Windows, to the
point that it doesn't even compile.  While it would be easy to
fix the compilation failure, this patch simply stops compiling
Editline entirely on Windows, as the simple compilation fix would
still result in a broken use of select on Windows, and as such a
broken implementation of Editline.

Since Editline was already disabled to begin with on Windows, we
don't attempt to fix the compilation failure or the underlying
issues, and instead just disable it "even more".

llvm-svn: 222177