Enea Zaffanella's fix for the PPCallbacks Elif callback, with a slight re-org, and an update of the new PPCallbacks test (soon to be moved to clang from extra), rather the unittest.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@196407 91177308-0d34-0410-b5e6-96231b3b80d8
Improve comment, as requested by Alp Toker.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@196409 91177308-0d34-0410-b5e6-96231b3b80d8