[llvm] [C API] Add blockaddress getters to C API (PR #77390)

[Benji Smith via llvm-commits]

Benjins wrote:

I made a new PR with the same API changes, but with a simpler unit test for them (modeling off other non-echo test functionality in `llvm-c-test`). Will close this one since it's now superseded
https://github.com/llvm/llvm-project/pull/81382

https://github.com/llvm/llvm-project/pull/77390