[llvm] [LLVM-C] Upstream `LLVMGetOrInsertFunction` from rustc (PR #162235)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

Can you please add an entry to llvm/docs/ReleaseNotes.md and add a proper PR description?

Otherwise this looks like. It's already possible to achieve this using LLVMAddFunction + LLVMGetNamedFunction, but getOrInsertFunction() is generally the more idiomatic way to insert declarations, so it makes sense to me to provide it directly.

https://github.com/llvm/llvm-project/pull/162235