Reapply [ConstantFold] Drop gep of gep fold entirely (#95126)

Reapplying without changes. The flang+openmp buildbot failure
should be addressed by https://github.com/llvm/llvm-project/pull/94541.

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This is a followup to https://github.com/llvm/llvm-project/pull/93823
and drops the DataLayout-unaware GEP of GEP fold entirely. All cases are
now left to the DataLayout-aware constant folder, which will fold
everything to a single i8 GEP.

We didn't have any test coverage for this fold in LLVM, but some Clang
tests change.