[lld] [LLD][COFF] Add support for MinGW auto-export on ARM64X (PR #125862)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> The decision to handle explicit exports separately for each symbol table is somewhat arbitrary, but it provides more flexibility. If users prefer a different behavior, they can explicitly control auto-exports using command-line arguments.
> 
> (Handling ARM64X in MinGW is still undecided, but I’m sending these changes now to avoid inconsistencies caused by silently ignoring them.)

Indeed, this case is quite unexplored yet...

Intuitively, it feels strange to autoexport symbols in one namespace but not in the other one. But code wise, it looks intuitive and straightforward in this form...

I guess the practical case is if we'd have e.g. a dllexport directive in one object file in one namespace, but none in the other one. Autoexporting all symbols on one side but only one single symbol in the other one feels a bit unexpected then, even if it may be logical. I wonder if it would be more intuitive if we'd end up with 1 symbol on one side and 0 on the other, in that case - what do you think?

In any case, I don't see any testcase added here?

https://github.com/llvm/llvm-project/pull/125862