Update HostProcess to use the same facade pattern as HostThread.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@217632 91177308-0d34-0410-b5e6-96231b3b80d8
Add missing constructor definition.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@217634 91177308-0d34-0410-b5e6-96231b3b80d8