[clang] [llvm] Adapted MemRegion::getDescriptiveName to handle ElementRegions (PR #85104)

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T-Gruber wrote:

Alright, I added a test case which leads to an incorrect index due to the use of getOriginRegion. I also added a FIXME comment to document that this case is not covered currently.

https://github.com/llvm/llvm-project/pull/85104