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

[jschear]

rules_swift's swift_library has a 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.