Implement -target-attach and -target-detach

Summary:
This changes add -target-attach and -target-detach. 

-target-attach allows lldb-mi to attach to an existing process by it's process id, matching gdb mi's syntax of '-target-attach <pid'. Additionally, support has been added for attaching to a process by name using '-target-attach -n <name>'. Combining this with --waitfor will allow lldb mi to attach to a process by name when the process starts. 

-target-detach simply detaches from the current process

Patch from chuckr@microsoft.com

Test Plan: I have added three tests, one each for -target-attach <pid>, -target-attach -n <name>, and -target-attach -n <name> --waitfor

Reviewers: paulmaybee, abidh, ChuckR

Subscribers: greggm, lldb-commits

Differential Revision: http://reviews.llvm.org/D9484

llvm-svn: 236705