[AsmPrinter] Make EmitLinkage and EmitVisibility public

Summary:
This allows target to implement custom emit of global variables if
required. See subsequent patch for a use case.

Change-Id: I9654197e3df24503104a54c41fff06845aed37fe

Reviewers: arsenm, kzhuravl

Subscribers: wdng, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61650

llvm-svn: 363519