[clangd] Improve code completion for ObjC methods

Summary:
Previously code completion did not work well for Objective-C methods
which contained multiple arguments as clangd did not expect to see
multiple typed-text chunks when handling code completion.

Note that even with this change, we do not consider selector fragments
from previous arguments to be part of the signature (although we
could in the future).

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: ilya-biryukov, ioeric, MaskRay, jkorous, arphaman, jfb, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D53934

llvm-svn: 346836