[llvm] Revert "[MC] Explicitly mark MCSymbol for MO_ExternalSymbol" (PR #133291)

[weiwei chen via llvm-commits]

weiweichen wrote:

> The AsmPrinter works on the general principle that everything should be representable in textual assembly: if you produce a .s file and then compile that to a .o file, that should be equivalent to producing the .o file directly.
> 
> To ensure that this works, the AsmPrinter can't modify MC datastructures directly; it has to make requests through the MCStreamer API. For example, if you want change symbol attributes, you have to go through MCStreamer::emitSymbolAttribute(). If you're emitting an object file directly, this will just modify the relevant datastructure... but if you're emitting assembly, it knows what directive to emit that has the equivalent effect.

I see! thank you for pointing this out, this makes sense. 

Would it be ok if I make the change through MCStreamer API, since [this comment](https://github.com/llvm/llvm-project/issues/132055#issuecomment-2749042715) seems to say that `MO_ExternalSymbol` should indeed be `extern`?

https://github.com/llvm/llvm-project/pull/133291