Otherwise, hits assertion, or crashes in no-assertion build.
Added 'EditableTextBuffer::getSize()' for getting size after previous updates
without actually applying them.
rdar://problem/34206143
This provides a very rudimentary way to check the end-to-end performance
of simple sourcekitd requests. A sample invocation might be
```
sourcekitd-test -time-request -repeat-request=10 -dont-print-response -req= ...
```
