In GDBRemoteCommunicationClient::GetHostInfo, don't set the

os to "ios" or "macosx" if it is unspecified.  For environments
where there genuinely is no os, we don't want to errantly 
convert that to ios/macosx, e.g. bare board debugging.

Change PlatformRemoteiOS, PlatformRemoteAppleWatch, and
PlatformRemoteAppleTV to not create themselves if we have
an unspecified OS.  Same problem - these are not appropriate
platforms for bare board debugging environments.

Have Process::Attach's logging take place if either 
process or target logging is enabled.

<rdar://problem/25592378> 

llvm-svn: 265732