Teach LLDB how to directly launch processes on the iOS simulator

This allows for command-line debugging of iOS simulator binaries (as long as UI is not required, or a full UI simulator has previously been otherwise launched), as well as execution of the LLDB test suite on the iOS simulator

This is known to compile on OSX 10.11 GM - feedback from people on other platforms and/or older versions of OSX as to the buildability of this code is greatly appreciated

llvm-svn: 252112