[IFUNC] Introduce GlobalIndirectSymbol as a base class for alias and ifunc

This patch is a part of http://reviews.llvm.org/D15525

GlobalIndirectSymbol class contains common implementation for both
aliases and ifuncs. This patch should be NFC change that just prepare
common code for ifunc support.

Differential Revision: http://reviews.llvm.org/D18433

llvm-svn: 265016