Xcode7 and later (based on some forum posts), the coverage system uses LLVM’s coverage generation and reporting mechanisms, the format for which is detailed at http://llvm.org/docs/CoverageMappingFormat.html. As of Xcode 9, this format does not support any means of exclusion of lines (or other structures).
The resulting mapping is exported into a consumable format (txt or html) by llvm-cov, which also doesn’t really have much in the way of exclusion mechanisms. llvm-cov does have some simple thresholding for only reporting on “greater than” or “less than” coverage for both lines and regions, but I suspect that’s not entirely what you’re after based on the question above.