Make StopInfoMachException return the right data. (#180088)

When I changed lldb so that the StopInfo's compute their own stop reason
data (before it was oddly done in SBThread::GetStopReasonData...) I
didn't notice that StopInfoMachException was relying on that routine's
default of returning the Value as the 0th exception data, and didn't
actually return its own data.
This fixes that, and makes us report the exception type, and the code
and subcode if the exception has them. I also added a test for this.

rdar://169755672

GitOrigin-RevId: 85d94e17144f2ca250c91b827b59e6ddea675d31
4 files changed