It's likely that these were listed as "REQUIRES: OS=macosx" to not
redundantly run them for iOS et al, but they don't take that long and
so it's more useful for Linux devs to be able to run them locally if
need be. Or to catch something that really is different on non-macOS.
This was causing an exponential amount of time traversing the AST with
deeply chained protocol extension methods, such as in the
TestCodableRouter.swift test in Kitura.
If the OpaqueValueExpr is referenced more than once within the
OpenExistentialExpr it'll still get visited more than once, but that
doesn't seem to happen in practice. If it turns out to be a problem,
we can weaken the assertion I'm adding here.
https://bugs.swift.org/browse/SR-11012
