[clang-format/ObjC] Use getIdentifierInfo() instead of tok::identifier

Summary:
Previously, we checked tokens for `tok::identifier` to see if they
were identifiers inside an Objective-C selector.

However, this missed C++ keywords like `new` and `delete`.

To fix this, this diff uses `getIdentifierInfo()` to find
identifiers or keywords inside Objective-C selectors.

Test Plan: New tests added. Ran tests with:
  % make -j16 FormatTests && ./tools/clang/unittests/Format/FormatTests

Reviewers: djasper, jolesiak

Reviewed By: djasper

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D46143

llvm-svn: 331067