[NFC] move GNUELFDumper::printEnum() into a common header for reuse.

Summary:
	This is a NFC patch moving the GNUELFDumper<ELFT>::printEnum()
 function from ELFDumper into ScopedPrinter.h for reuse.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D114840