Clean up some existing keyword tests in the test/Lexer directory by using the
pre-defined __is_identifier() macro.

Differential Revision: http://reviews.llvm.org/D3460



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208147 91177308-0d34-0410-b5e6-96231b3b80d8