[llvm] [llvm-c] Add C API methods to match size_t ConstantDataArray C++ API signatures (PR #84433)

[Nikita Popov via llvm-commits]

nikic wrote:

Can you please also add a note in https://github.com/llvm/llvm-project/blob/main/llvm/docs/ReleaseNotes.rst#changes-to-the-c-api?

https://github.com/llvm/llvm-project/pull/84433