Add new ivars to StackFrame so it can represent a stack collected
at some point in the past.  We may have nothing more than a pc value
for this type of stack frame -- hopefully we'll have a pc and a
stop_id so we can track module loads and unloads over time and
symbolicate the pc at the correct point in time.

Also add a flag to indicate if the CFA for the frame is available
(a bit different from a CFA of LLDB_INVALID_ADDRESS) and also an
overall setting to indicate whether this is a history stack frame
or not.  A history stack frame may not have a CFA, it may not have
a register context, it may not have variables, it may not have a
frame pointer or a stack pointer.

<rdar://problem/15314068>