[clang] [clang][macos] support `__attribute__((weak, alias("...")))` on darwin (PR #211539)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> > Why do we not also support strong aliases?
> 
> I'm actually not sure. My read is that it was done because GCC did it, but GCC on apple platforms hasn't really been a thing for a long time apparently. Based on some brief experiments strong aliases do seem to work.

Then can you also allow that? You should do some checking that after the weak alias is overridden by a different definition, the private symbol is not affected.

https://github.com/llvm/llvm-project/pull/211539