[clang] [sanitizer] [clang] Introduce fsanitize-bounds-strict-flex-arrays (PR #126163)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

What else even uses the level?  __builtin_object_size codegen?  Can you add a DocBrief to the flag explaining when you'd want to use it?

https://github.com/llvm/llvm-project/pull/126163