The back-deployed Swift Concurrency library should not contain anything
based on clocks or durations, which aren't always available in the
underlying system. Move that functionality to separate files that are
excluded from the back-deployed concurrency libraries.
This is a partial step to better approximate the back-deployment
libraries. At some point, we'll stop building the back-deployment
libraries entirely and instead use the binaries provided by the
toolchain.
Fixes rdar://89237163.