[lld] [llvm] [LLD][COFF] Allow overriding EC alias symbols with alternate names (PR #113456)

[Jacek Caban via llvm-commits]

cjacek wrote:

More experimentation with MSVC revealed that `-alternatename` is effective on anti-dependency symbols if either the aliased symbol is undefined or the symbol is mangled. In practice, this distinction helps differentiate between an alias to the guest exit thunk (which is always mangled) and a demangled alias to the implementation symbol. I kept it compatible; it’s now ready for review.

https://github.com/llvm/llvm-project/pull/113456