cmake: split UB sanitations into separate flags.

Clang undefined behaviour sanitizer was turned on using
-fsanitize=undefined flag, which is supposed to turn on all the
sanitizations, except a few ones. Not needed sanitations were
turned off explicitly, using -fno-sanitize=<type> flags. However
appeared it does not work with some flags. For example,
nullability sanitations can't be turned off when
-fsanitize=undefined is used.

Nullability sanitations lead to lots of false-positive fails
such as typeof(*obj) where obj is NULL, or memcpy() with NULL
destination but 0 size.

The patch splits -fsanitize=undefined into separate flags and
never turns on nullability checks.

Part of #4609