CodeGen: Cleanup variable linkage calculation

Almost all linkage calculation for VarDecls occured inside of
GetLLVMLinkageVarDefinition except for static data members.  Centralize
the logic so that it can be more readily reused.

No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207241 91177308-0d34-0410-b5e6-96231b3b80d8