Allow users to simultaneously enable address and undefined behavior
sanitizers, in the same manner that LLVM's 'HandleLLVMOptions.cmake'
Prior to this patch, cmake -DLLVM_USE_SANITIZER="Address;Undefined"
would succeed and the build would build most of the LLVM project with
-fsanitize=address,undefined, but a warning would be printed by
libcxx's CMake, and the build would use neither sanitizer. This
patch results in no warning being printed, and both sanitizers are used
in building libcxx.