[Lexer] Ensure that the token is not an annotation token when
retrieving the identifer info for an Objective-C keyword

This commit fixes an assertion that's triggered in getIdentifier when the token
is an annotation token.

rdar://32225463

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@303246 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit bd77dc6b8f04ac9afcb5487e91d7a5034521e2cc)
4 files changed