[SelectionDAG] Rename CallSiteDbgInfo to NodeExtraInfo

For information infrequently attached to SDNodes, it is useful to
provide a way to add this information out-of-line. This is already done
for call-site specific information.

Rename CallSiteDbgInfo to NodeExtraInfo in preparation of adding
additional information not necessarily related to call sites only.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D130880