`CIncludeDir` is unused

[dschrempf]

CIncludeDir is (was?) used in hs-bindgen, but not in libclang since we simplified ClangArgs.

The documentation of CIncludeDir, however, contains valuable information which should not be lost.

Maybe we should move the declaration to hs-bindgen? (FYI, I am in the middle of refactoring ClangArgsConfig to be polymorphic over the file path).

[edsko]

@TravisCardwell Do you have an opinion here?

[TravisCardwell]

Indeed, CIncludeDir is no longer used in libclang. In hs-bindgen, we are simply using FilePath instead of this type.

Moving the type into hs-bindgen and using it instead of FilePath sounds good to me. 👍