Add ABSL_SWISSTABLE_ASSUME_SEPARATE_STORAGE and use it in raw_hash_set::control().

Apparently, the manual assume we had with pointer values wasn't doing anything (we previously thought it was valuable but apparently the original change's codegen effects were side effects of e.g. calling `NumControlBytes(capacity())`).

Benefits of the new `ABSL_SWISSTABLE_ASSUME_SEPARATE_STORAGE(ctrl, this)` in `raw_hash_set::control()`:

* Unlike the previous `reinterpret_cast<uintptr_t>` range comparison inside `ABSL_ASSUME` (which LLVM's `BasicAA` ignores), `__builtin_assume_separate_storage(ctrl, this)` lowers to an `@llvm.assume` `"separate_storage"` operand bundle that `BasicAA` directly uses to prove that pointers derived from `ctrl` (including `slot_array` and iterator slot pointers on the non-SOO path) cannot alias `*this` (or any enclosing object containing the table).

* When a caller mutates an element via an iterator returned by `find()` or `begin()` (e.g. `*it = v` or `it->second = v`), the compiler can now prove that the store to the slot does not clobber `CommonFields` metadata (`size`, `capacity`, `seed` in `0(%rdi)`), eliminating redundant reloads of `CommonFields` (and freeing `this` earlier to reduce register pressure) across subsequent `size()`, `capacity()`, or `find()` operations.

Benefits of removing the previous assume from `CommonFields::control()` on `raw_hash_set_benchmark` (`--config=benchmark --cpu=haswell`, `llvm-mca -mcpu=znver3`):

* `raw_hash_set::find`: Removing `NumControlBytes(capacity())` from `CommonFields::control()` allows `prefetcht2` (`prefetch_heap_block()`) to be scheduled immediately after loading `ctrl` (`movq 8(%rdi), %rax`), ahead of the capacity mask setup (`movq $-1, %r8; shlxq %rdx, %r8, %rcx`).

* Under `llvm-mca -mcpu=znver3 -iterations=1`, this promotes `prefetcht2` from the second dispatch group (cycle 1) into the first dispatch group (cycle 0) while leaving static L1-hit latency unchanged (36 cycles for `CodegenAbslRawHashSetInt64Find`, 37 cycles for `CodegenAbslRawHashSetInt64FindNeEnd`, 143 cycles for `StringTable::find`), and avoids a stack spill in `DescriptorByNameHash` `raw_hash_set::find<string_view>` (133 → 131 cycles, RThroughput 29.7 → 29.3).

* `raw_hash_set.cc` (`CommonFields`): Removing `reinterpret_cast<uintptr_t>(this)` from `CommonFields::control()` allows LLVM argument promotion on `ReportInsertMissToInfoz` (11 → 8 instructions, 15 → 10 cycles at `-iterations=1`) and slightly reduces instruction counts in `ResizeAllocatedTableWithSeedChange` (377 → 374 instructions, 244 → 243 cycles) and `EraseMetaOnlyLarge` (78 → 77 instructions, RThroughput 13.7 → 13.3).

PiperOrigin-RevId: 1003430109