[clang-format][NFC] Add FormatToken::is(tok::ObjCKeywordKind) (#134973)
This allows simplification of code that checks if a token is an
Objective-C keyword.
Also, delete the following in
UnwrappedLineParser::parseStructuralElement():
- an else-after-break in the tok::at case
- the copypasted code in the tok::objc_autoreleasepool case