Fix =thread-exited message (MI)

Summary:
This patch includes:
# Fix invalid thread id in =thread-exited message
# Remove invalid threads from cache

All tests pass on OS X.

Test Plan:
now
```
=thread-exited,id="3",group-id="i1"
=thread-exited,id="4",group-id="i1"
=thread-exited,id="5",group-id="i1"
```

was
```
=thread-exited,id="4294967295",group-id="i1"
=thread-exited,id="4294967295",group-id="i1"
...
=thread-exited,id="4294967295",group-id="i1"
=thread-exited,id="4294967295",group-id="i1"
```

Reviewers: abidh

Reviewed By: abidh

Subscribers: lldb-commits, abidh

Differential Revision: http://reviews.llvm.org/D8603

llvm-svn: 233256