[llvm][AsmPrinter][NFC] Cleanup `GCMetadataPrinters` field

The field is currently `void*`, which was originlly chosen in 2010 to not need to include `DenseMap`. Since then, `DenseMap` has been included in the header file anyways, so there is no more need to for the indirection via `void*` and the cruft around it can be removed.

Differential Revision: https://reviews.llvm.org/D140758