`InterfaceSubContextDelegateImpl` causes sub-instances to inherit `-fmodule-map-file=` options.
Those Module Maps become file dependencies of all downstream PCMs and their depending Swift modules, even though they really aren't.
This causes frequent re-builds of the Module Cache contents when seemingly-unrelated files are touched.
Explicit Module Builds rely on these options for building Swift Interface files, so for now we just disable inheritance of these options in Implicit Module builds.
