libcxx 'LLVM_USE_SANITIZER=Address;Undefined'

Summary:
Allow users to simultaneously enable address and undefined behavior
sanitizers, in the same manner that LLVM's 'HandleLLVMOptions.cmake'
allows.

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.

Reviewers: jroelofs, EricWF, ldionne, #libc!

Subscribers: mgorny, dexonsmith, llvm-commits, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D77466