Reland "[PA] Refactor to use a typed SlotStart to replace uintptr_t"

This reverts commit dff40e5d80fe0e2afd678288d6c7aebd33b0e80d.

Reason for revert: Failure fixed; see the diff from PS1.

Original change's description:
> Revert "[PA] Refactor to use a typed SlotStart to replace uintptr_t"
>
> This reverts commit 2b12f09a50acd7d19a7cd07db00a10c00c1297c1.
>
> Reason for revert: the changed tests in SlotStartTest are failing for multiple bots (sample https://ci.chromium.org/ui/p/chrome/builders/ci/linux-chromeos-chrome and https://ci.chromium.org/ui/p/chromium/builders/ci/mac15-x64-rel-tests)
>
> Original change's description:
> > [PA] Refactor to use a typed SlotStart to replace uintptr_t
> >
> > This CL refactors PartitionAlloc to use a new SlotStart type instead of
> > raw `uintptr_t` to represent the start of a slot. This improves type
> > safety and makes the code easier to understand and maintain.
> >
> > The new SlotStart class has two variants: SlotStart (tagged) and
> > UntaggedSlotStart. It also provides Checked and Unchecked factory
> > methods. The Checked variant performs a runtime check to ensure that the
> > given address is indeed a slot start, which helps catch memory
> > corruption bugs earlier.
> >
> > The following changes were made:
> >    - Introduced SlotStart, UntaggedSlotStart, and SlotSpanStart classes.
> >    - Replaced most usages of raw uintptr_t for slot starts with the new
> >      types.
> >    - Replaced PartitionRoot::ObjectToSlotStart and
> >      PartitionRoot::SlotStartToObject with `SlotStart::Checked` and
> >      `SlotStart::ToObject`.
> >    - Updated numerous call sites to use the new SlotStart API family.
> >
> > This change is a pure refactoring and should not have any functional
> > impact.
> >
> > Cq-Include-Trybots: luci.chromium.try:android-mte-arm64-rel
> > Change-Id: Ide9c8ff7dfd77d226ba806fa0f6f4a75f2e30ff9
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7125918
> > Reviewed-by: Sergei Glazunov <glazunov@google.com>
> > Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
> > Auto-Submit: Mikihito Matsuura <mikt@google.com>
> > Commit-Queue: Sergei Glazunov <glazunov@google.com>
> > Reviewed-by: Takashi Sakamoto <tasak@google.com>
> > Cr-Commit-Position: refs/heads/main@{#1543027}
>
> Cq-Include-Trybots: luci.chromium.try:android-mte-arm64-rel
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Change-Id: I7e88b353ce7f1dfb37e82dcf2d77001f56693cce
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7141579
> Commit-Queue: Jood Hajeer <jood@google.com>
> Reviewed-by: Ankush Singh <ankushkush@google.com>
> Owners-Override: Jood Hajeer <jood@google.com>
> Cr-Commit-Position: refs/heads/main@{#1543080}

Change-Id: Ia725f5665d38018677b811164483c1df6196fb92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7140801
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Sergei Glazunov <glazunov@google.com>
Reviewed-by: Takashi Sakamoto <tasak@google.com>
Commit-Queue: Takashi Sakamoto <tasak@google.com>
Cr-Commit-Position: refs/heads/main@{#1544717}
NOKEYCHECK=True
GitOrigin-RevId: 47a14fbc9b59a0e847c7f941719c4d83cc61eaaa

clang_code_coverage_wrapper.py

diff --git a/clang_code_coverage_wrapper.py b/clang_code_coverage_wrapper.py
index 9b94bcf..807b6ec 100755
--- a/clang_code_coverage_wrapper.py
+++ b/clang_code_coverage_wrapper.py
@@ -217,6 +217,7 @@
     '../../base/allocator/partition_allocator/src/partition_alloc/shim/malloc_zone_functions_apple_unittest.cc',  #pylint: disable=line-too-long
     '../../base/allocator/partition_allocator/src/partition_alloc/shim/winheap_stubs_win.cc',  #pylint: disable=line-too-long
     '../../base/allocator/partition_allocator/src/partition_alloc/shim/winheap_stubs_win_unittest.cc',  #pylint: disable=line-too-long
+    '../../base/allocator/partition_allocator/src/partition_alloc/slot_start.cc',  #pylint: disable=line-too-long
     '../../base/allocator/partition_allocator/src/partition_alloc/slot_start_unittest.cc',  #pylint: disable=line-too-long
     '../../base/allocator/partition_allocator/src/partition_alloc/spinning_mutex.cc',  #pylint: disable=line-too-long
     '../../base/allocator/partition_allocator/src/partition_alloc/stack/asm/arm64/push_registers_asm.cc',  #pylint: disable=line-too-long