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

[Balazs Benics via cfe-commits]

steakhal wrote:

I hope you don't mind that I reworked the testing a bit, to be similar to the rest of the tests.
I also reverted unnecessary clang-formatting where it was not relevant.
I refined the `auto` usage, but other than that I left your code untouched.

As I did everything I thought, I'll invite others to review the PR.

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