Refactor swisstable iterator debug messages code. The motivations are (a) distinguish between the "likely erased" and "could have rehashed" cases when generations are enabled, (b) suggest running under ASan when generations aren't enabled and doing so would narrow down the possible error cases, and (c) make ABSL_INTERNAL_ASSERT_IS_FULL not be a macro.
PiperOrigin-RevId: 511255275
Change-Id: I5a44a813cd310837d0bd0209d2187b100be201e7
