swift234

[CMake] Make it an error to build without having the PerfTests checked out.

Per a push from Nadav and JoeP to make the perf tests easier to run, or at
the very least harder to break. If you're changing the standard library API,
please build PerfTests_Onone before committing. If you're doing something
that might affect performance, please build and run all three PerfTest targets
(_Onone, _O, and _Ounchecked) and make sure that you haven't introduced any
unexpected regressions.

There is still a build setting to disable this error, but that's only
intended for the buildbots to use.

Swift SVN r22485