[clang-format/ObjC] Fix counting selector name parts for ObjC

Summary:
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

llvm-svn: 336518