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

[Florian Mayer via cfe-commits]

fmayer 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?

Let me find out details. We have some code that miscompiles with `-fstrict-flex-arrays=1`, so something is exploiting the UB. 

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