[Parser][ObjC] Use an artificial EOF token while parsing lexed ObjC methods

This change avoid a crash that occurred when skipping to EOF while parsing an
ObjC interface/implementation.

rdar://31963299

Differential Revision: https://reviews.llvm.org/D34185


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@305719 91177308-0d34-0410-b5e6-96231b3b80d8