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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

I'm not sure what the actual goal of the original patch is.  If a symbol is not defined, it defaults to being external.  If a symbol is defined, we emit the directives for the definition at the point of the definition.  So I'm not sure what effect you're hoping for, besides breaking Julia's hack.

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