[clang] [analyzer] Check the correct first and last elements in cstring.UninitializedRead (PR #95408)

[Donát Nagy via cfe-commits]

https://github.com/NagyDonat requested changes to this pull request.

Thanks for the updates!

Unfortunately, as I dug deeper into your function `getOriginRegion()`, I realized that it is logically incorrect (see inline comment for explanation). (I was a bit suspicious during the earlier reviews as well, but this is a very complex area and earlier I didn't spend enough time on it.)

https://github.com/llvm/llvm-project/pull/95408