Handle LLVM_USE_SANITIZER=Address;Undefined (and the other way around)

Summary:
Handle LLVM_USE_SANITIZER=Address;Undefined to enable ASan and UBSan
If UBSan is compatible with more of the other sanitizers, maybe we should
deal with this in a better way where we allow combining UBSan with any of
the other sanitizers.

Reviewers: samsonov

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D7024

llvm-svn: 228219