`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).