Enable -Wundef.

The problem that caused the need for http://reviews.llvm.org/D7419 was
caused by testing the value of something that was undefined. This
should prevent that in the future.


git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@228257 91177308-0d34-0410-b5e6-96231b3b80d8
16 files changed