Fix MacOSX Xcode build for Pipe changes.

Also cleans up the lldb Xcode project's Source > Host > posix directory
to use group-relative file naming.  Also alphabetizes file entries.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@219373 91177308-0d34-0410-b5e6-96231b3b80d8