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

[Oliver Hunt via llvm-commits]

ojhunt wrote:

These need to be pulled into distinct PRs - you can make chained PRs if you feels there's sufficient dependencies from one to another, but a good rule of thumb is that if you're adding multiple entries into the release notes it needs different PRs.

As it is this PR is too large to be reasonably reviewed - I'll do a _very_ quick pass, but until it's split appropriately there's not much chance of getting this landed.

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