Add a `cc_inputs` attribute to include arbitrary files in compilation actions and support location expansion

`rules_swift`'s `swift_library` has a [swiftc_inputs](https://github.com/bazelbuild/rules_swift/blob/master/doc/rules.md#swift_library-swiftc_inputs) attribute, which serves as a way to pass labels that are used in `$(location)` expansion in other attributes.

There are a few places in `rules_swift` and `rules_apple` where the `headers` field of `CcInfo` is being used/abused for this purpose:
- https://github.com/bazelbuild/rules_apple/blob/master/apple/internal/header_map.bzl#L98
- https://github.com/bazelbuild/rules_swift/blob/646d6d229edc0b22ea3b9dc891062de519d27d4c/mixed_language/internal/library.bzl#L154

It would be nice if there was a dedicated attribute. I'd be happy to contribute if this request makes sense.