[lld] [LLD][COFF] Add support for importing ARM64X images (PR #210080)

[Martin Storsjö via llvm-commits]

https://github.com/mstorsjo approved this pull request.

Code wise, this looks great, thanks! There were some more nonobvious details to this than I had expected.

The commit message is a bit hard to understand if one isn't directly familiar with this feature though - "importing an image" sounds odd, and it's not directly clear that this is something different than the usual of using an import library.

What about something like this?

```
[LLD] [COFF] Fix linking directly against an ARM64X DLL without import library

In mingw mode, when linking against a DLL, the user can either provide a regular import library, or provide the actual DLL.

[... rest of existing commit message ...]
```

https://github.com/llvm/llvm-project/pull/210080