Emit column debug information for loads

This allows us to give more precise diagnostics.

Diego kindly tested the impact on debug info size: "The increase on average
debug sizes is 0.1%. The total file size increase is ~0%."

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@214162 91177308-0d34-0410-b5e6-96231b3b80d8