[llvm] [llvm][aarch64] Fix Arm64EC name mangling algorithm (PR #115567)

[Daniel Paoliello via llvm-commits]

dpaoliello wrote:

> Do you think it makes sense to cherry-pick this to 19.x?

Yeah, that's probably a good idea otherwise we'll be generating different names than MSVC

https://github.com/llvm/llvm-project/pull/115567