PGO: Rename FuncLinkage to VarLinkage; no functionality change

The variable is used to set the linkage for variables, and will become
different from function linkage in a follow-up commit.

<rdar://problem/15943240>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204407 91177308-0d34-0410-b5e6-96231b3b80d8