Add more owners to CODE_OWNERS.TXT (Kostya Serebryany: AddressSanitizer and ThreadSanitizer; Evgeniy Stepanov: MemorySanitizer)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185064 91177308-0d34-0410-b5e6-96231b3b80d8