swift234

Reapply "Fix perf test suite option in build-script-impl. Now it /should/ work."

Was c3ed3c3aeca4ee1756b2a6ad3ebeb35d96dbecb4.

This makes us honour the value of the setting, not just the presence of
it, and in particular it makes the default of disabling it work.

rdar://problem/23524451