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

[Florian Mayer via cfe-commits]

fmayer wrote:

> Usually our tweak flags looks like -fsanitize--
> 
> "bounds" is a group for (array-bounds + local-bounds) "undefined" is also group, but we already have -fsanitize-undefined-, but they kind of affect a few sanitizers
> 
> so: `-fsanitize-undefined-strict-flex-arrays` or `-fsanitize-array-bounds-strict-flex-arrays` ?

done

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