We plan to replace the existing mechanism for overriding detection with
one that doesn't require the use of a special section as an alternative
to llvm/llvm-project#120805 which had other downsides.

This change is a pure refactoring that lays the foundation for a
subsequent change that will introduce the new detection mechanism.

Reverts llvm/llvm-project#120805

This change while desirable has two issues we discovered:

- It is incompatible with `-funique-internal-linkage-names`, see
https://github.com/llvm/llvm-project/pull/120805#discussion_r1913709817
- It is incompatible with `-fvisibility-global-new-delete=force-hidden`,
see
https://github.com/llvm/llvm-project/issues/123224#issuecomment-2607963878

We were hoping to address both of these issues with
https://github.com/llvm/llvm-project/pull/122983, but that change has
other issues we haven't yet managed to resolve. For now, we have decided
to revert the change to avoid shipping a broken feature in LLVM 20, and
we plan to follow up with a new approach post branch.

This mechanism is preferable in environments like embedded since it
doesn't require special handling of the custom section.

This is a reland of https://github.com/llvm/llvm-project/pull/114961
which addresses the issue reported by downstream users. Specifically,
the two differences from the previous version are:

* The internal `symbol##_impl__` symbol in the Mach-O implementation is
  annotated with `__attribute__((used))` to prevent LTO from deleting it
  which we've seen in the previous version.
* `__is_function_overridden` is marked as `inline` so these symbols are
  placed in a COMDAT (or fully inlined) to avoid duplicate symbol errors
  which we've seen in the previous version.

This reverts commit 62bd10f7d18ca6f544286767cae2c9026d493888.
Breaks building with -flto=thin, see
https://github.com/llvm/llvm-project/pull/114961#issuecomment-2555754056

This mechanism is preferable in environments like embedded since it
doesn't require special handling of the custom section.

Apparently, there are two almost identical implementations: one for
MachO and another one for ELF. The ELF bits somehow slipped while
https://github.com/llvm/llvm-project/pull/84573 was reviewed.

The particular implementation is identical to MachO case.

Summary:
The logic for this `__is_function_overridden` check requires accessing a
runtime array normally created by the linker. The NVPTX target is an
`__ELF__` target, however it does not support emitting the
`__start/__stop` symbols for C-identifier named sections. This needs to
be disabled explicitly so that the user can compile this with anything.

This patch applies the comments provided on #84573. This is done as a
separate PR to avoid merge conflicts with downstreams that already had
ptrauth support.