[llvm] [IR][ARM64EC][NFC] Clean up and document ARM64EC mangling helpers. (PR #107230)

[Jacek Caban via llvm-commits]

cjacek wrote:

This PR addresses @mstorsjo's comment from #107164. Since the C++ and non-C++ code paths don't share much beyond the return statement, I've separated them further. This change eliminates the need for the Prefix variable (which triggered the original comment) and makes the non-C++ case easier to follow. I also added some comments for clarity.

https://github.com/llvm/llvm-project/pull/107230