Added a new info.plist key `CDInheritDefaultAvailability` that allows customization on DocC logic for using default availability as the availability information for symbols.

`CDInheritDefaultAvailability` allows two values:
- `platformAndVersion` (deafault): This value adds both the platform and the semantic version as a fallback value for symbols that don't define explicit availability information for the given platform.
- `platformOnly`: This value only adds the platform name (removes the version) to the symbols that don't define an explicit availability annotation for the given platform.

rdar://132980711

`DefaultAvailabilityOptions` is a new info plist key that contains a dictionary of options to customise the logic of the default availbaility.

The only option for now is `InheritVersionNumber` that lets you add the version into the symbols availability that don't define explicit availability annotation. The default is true.

…lt availability options.