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

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228219 91177308-0d34-0410-b5e6-96231b3b80d8