[clang] [llvm] [Clang] Correct __builtin_dynamic_object_size for subobject types (PR #78526)

[Richard Smith via llvm-commits]

https://github.com/zygoloid commented:

We've discussed this before, years ago. Previously, the sticking point was: how is LLVM going to know what the frontend considers the closest surrounding subobject to be? The LLVM type doesn't give you that information, and it seems like there's nowhere else that LLVM can get it from either, so this flag ends up not being useful and the best we can do is to give a conservatively-correct answer -- the complete object size if we want an upper bound, or 0 if we want a lower bound.

Clang does respect the "subobject" flag if it can symbolically evaluate the operand of `__builtin_object_size` sufficiently to determine which subobject is being referenced. Previously we've thought that that was the best we could do.

https://github.com/llvm/llvm-project/pull/78526