Move ConnectionFileDescriptor to platform-specific Host directory.

As part of getting ConnectionFileDescriptor working on Windows,
there is going to be alot of platform specific work to be done.
As a result, the implementation is moving into Host.  This patch
performs the code move and fixes up call-sites appropriately.

Reviewed by: Greg Clayton
Differential Revision: http://reviews.llvm.org/D5548

llvm-svn: 219143