Add support for handling ifuncs to GlobalValue::getBaseObject

Summary:
All GlobalIndirectSymbol types (not just GlobalAlias) should return
their base object.

Without this patch LTO would warn "Unable to determine comdat of
alias!" for an ifunc.

Reviewers: pcc

Subscribers: mehdi_amini, inglorion, llvm-commits

Differential Revision: https://reviews.llvm.org/D33202

llvm-svn: 303096