[libcxx] Add support for LLVM_USE_SANITIZER=Undefined

LLVM_USE_SANITIZER=Undefined support was added to the LLVM CMake configuration.
Update libc++'s handling of LLVM_USE_SANITIZER to support this as well.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@219987 91177308-0d34-0410-b5e6-96231b3b80d8