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
The code goes into its own sub-tree under 'tools' but tests go under 'test',
so that running 'check-swift' will also run all the SourceKit tests.
SourceKit is disabled on non-darwin platforms.