[lldb] [Core] Harmonize Communication::Read() returns w/ thread

Harmonize the status and error values of Communication::Read() when
running with and without read thread.  Prior to this change, Read()
would return eConnectionStatusSuccess if read thread was enabled
and the read timed out or reached end-of-file, rather than
the respective states that are returned if read thread was disabled.
Now, it correctly returns eConnectionStatusTimedOut
and eConnectionStatusEndOfFile, and sets the error respectively.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.llvm.org/D132217