Fix MSVC build

Added _WIN32 guards to new platform features. Using correct SetErrorStringWithFormat within Host when LLDB_DISABLE_POSIX is defined. Also fixed an if defined block.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@195766 91177308-0d34-0410-b5e6-96231b3b80d8