Found by TSan!
This was found while trying to reproduce a test failure on a linux bot
while running the test/Misc/stats.swift test. Hopefully this was the
underlying issue.
rdar://35537968
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.