Add UBSAN annotation to __hash_table::rehash; we don't do anything wrong, but...

Add UBSAN annotation to __hash_table::rehash; we don't do anything wrong, but UBSAN's checker flags it as suspicious. See PR38606. NFC

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@353448 91177308-0d34-0410-b5e6-96231b3b80d8