IR: Add DbgInfoIntrinsic::getVariableLocation

Create a common accessor, DbgInfoIntrinsic::getVariableLocation, which
doesn't care about the type of debug info intrinsic.  Use this to
further unify the implementations of DbgDeclareInst::getAddress and
DbgValueInst::getValue.

Besides being a cleanup, I'm planning to use this to prepare DEBUG
output without having to branch on the concrete type.

llvm-svn: 264767