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

[Nikita Popov via llvm-commits]

nikic wrote:

> Would it be a reasonable goal to update all of these length methods & types to be uint64_t, or would that be too breaking?

Can't say this is in general, but at least updating ConstantDataSequential::getNumElements() to use uint64_t sounds like a good idea.

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