[clang] [Clang] Convert __builtin_dynamic_object_size into a calculation (PR #80256)

[Bill Wendling via cfe-commits]

bwendling wrote:

This is a first step in improving our `__bdos` implementation. It doesn't address the "sub-object" discussion from the previous PR (https://github.com/llvm/llvm-project/pull/78526), that will come later on. This patch takes care of a common use of `__bdos` that can't be calculated by the front-end (i.e. performing a __bdos on an array with an variable index). I'll be expanding the cases this can handle in the future.

https://github.com/llvm/llvm-project/pull/80256