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

[Balazs Benics via cfe-commits]

steakhal wrote:

I checked out the code and gave it a test run. Across 200+ projects, it suppresses around 650 reports. I've sampled them and they appear like the FP this patch was motivated by.
I don't really have the time to do the review, as I'll leave for vacation, but the intent of the patch and the overall effect of the patch is aligned with my expectation. @balazske has extensive experience w.r.t. buffers and element regions - which is a tricky area IMO, but the PR should be in good hands, including you.

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