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

[via llvm-commits]

erer1243 wrote:

@DianQK I amended the commit to use a regular name & email - is that ok?

Also removed LLVMConstString2 and added a release note.

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