[clang] Improve C string null terminator diagnostics (PR #190411)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

Please split warnings with different implementation strategies into separate patches.  The warnings here are "related" in some sense, but the actual implementation logic isn't connected.  (And sometimes we want different reviewers depending on what part of the codebase you're touching.)

Since this is your first PR, maybe just start with warn_fortify_literal_copy_too_large.

https://github.com/llvm/llvm-project/pull/190411