Add some unconditional logging on the failure points when attaching

Add some unconditional logging on the failure points when attaching
to a process so we'll always get messages in the console logs.

Also make the "is frontboard process" / "is backboard process"
determination lazy, specifically take it out of the
MachProcess::AttachForDebug codepath when we are attaching to a
process, to simplify attaching.

<rdar://problem/47982516>
<rdar://problem/48060134>

llvm-svn: 354181