[clang] [clang] Add unistd fortify checks and analyzer size constraints (PR #196499)

[Denys Fedoryshchenko via cfe-commits]

nuclearcat wrote:

@venk-ks I’ve split this into #224979 (Sema) and #224980 (analyzer). While reviewing the split, I found two issues -lookup overhead and oversized-count warnings stopping analysis, that I’ll address today. If you’d prefer a different split, please let me know; you’re more familiar with this area, and I’m happy to adjust or hand over.

https://github.com/llvm/llvm-project/pull/196499