Do not warn on keyword undef

#undef a keyword is generally harmless but used often in configuration scripts.
Also added tests that I forgot to include to commit in r223114.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224100 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed