[lldb] [ConnectionFileDescriptorPosix] Use a single NativeFile

Replace separate read and write NativeFile instances with a single
instance shared for reading and writing.  There is no clear indication
why two instances were used in the first place, and replacing them
with just one does not seem to cause any regressions in tests or manual
'process connect file://...'.

Differential Revision: https://reviews.llvm.org/D111314