Revert "[libcxx] Use alias for detecting overriden function" (#124431)

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.

GitOrigin-RevId: 4167ea2cb082a2acb00b8b1dc09aa780dc0e3110
2 files changed