Centralized the launching of a process into Target::Launch()

While investigating test suite failures when running the test suite remotely, I noticed we had 3 copies of code that launched a process:
1 - in "process launch" command 
2 - SBTarget::Launch() with args
3 - SBTarget::Launch() with SBLaunchInfo

"process launch" was launching through the platform if it was supported (this is needed for remote debugging) and the 2 and 3 were not.

Now all code is in one place.



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@197247 91177308-0d34-0410-b5e6-96231b3b80d8