Cache config/setting file misses too

[kastiglione]

SettingsBuilder.swift has two caches for BuildSettings, for xcconfigs and settings presets. The caches ensure existing files aren't redundantly reloaded, but for non-existent files they don't cache the fact that no settings file could be found, which means redundant searches for any optional settings file that does not happen to exist. Each failed search is repeated work. For the project I'm working on, this is most expensive in SettingsPresetFile.getBuildSettings().

This change makes these caches also cache which config files do not exist. For just getBuildSettings(), this change reduces the memory use, down to 1.7mb, from a little over 10mb. The sample project has ~40 targets but real world targets for us will be bigger.

This change adds an Optional equivalent enum named Cached. This is to avoid having a dictionary value of an Optional type, otherwise the subscript operator would return an Optional<Optional<T>>, which can be awkward to work with.

[yonaskolb]

Great stuff, thanks @kastiglione!