[clang-format/ObjC] Fix counting selector name parts for ObjC
Counts selector parts also for method declarations and counts correctly for methods without arguments.
This is an internal change and doesn't influence formatting on its own (at the current state). Its lack would be visible after applying D48719.
Reviewers: benhamilton, klimek
Reviewed By: benhamilton
Subscribers: acoomans, cfe-commits
Differential Revision: https://reviews.llvm.org/D48716