Add some more comments explaining the purpose of some Register classes.
Disable GetSTDOUT, GetSTDERR, and PutSTDIN on Windows.

These methods are difficult / impossible to implement in a way
that is semantically equivalent to the expectations set by LLDB
for using them.  In the future, we should find an alternative
strategy (for example, i/o redirection) for achieving similar
functionality, and hopefully deprecate these APIs someday.
When a process stops, set the StopInfo object on Windows.