Added a test case that verifies that LLDB can debug across a process exec'ing...

Added a test case that verifies that LLDB can debug across a process exec'ing itself into a new program. This currently is only enabled for Darwin since we exec from 64 bit to 32 bit and vice versa for 'x86_64' targets.

This can easily be adapted for linux and other platforms, but I didn't want to break any buildbots by assuming it will work.

llvm-svn: 182428