[patch] Avoid a Symbol -> Name -> Symbol conversion

[Rafael Espíndola]

Currently some code produces an TargetExternalSymbol from a MCSymbol.
That means extracting the symbol name and fetching the symbol again
down the pipeline.

The attached patch adds a DAG.getMCSymbol that lets the MCSymbol pass
unchanged on the DAG.

Doing so removes the need for MO_NOPREFIX and fixes the root cause of
pr23900, allowing r240130 to be committed again.

Cheers,
Rafael
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