[lld] [llvm] [Object][COFF] Avoid underscore prefix for forwarding exports (PR #135433)

[Jacek Caban via llvm-commits]

cjacek wrote:

Exporting names with dots is a bit unusual, but MSVC’s lib.exe allows it. So, comparing to the original version I posted in the issue, I’ve added a check that `ExtName` is not empty, meaning it's the forward name, not the export name. I also added a test for that.

CC @lhmouse

https://github.com/llvm/llvm-project/pull/135433