[NFC][SelectionDAG][DebugInfo] Refactor DanglingDebugInfo class

Hide the underlying DbgValueInst by adding methods to extract the necessary
information and by adding a raw_ostream &operator<< overload to print it.

Remove the DebugLoc field as this is always the same as the DbgValueInst's
DebugLoc (see D136247).

Reviewed By: StephenTozer

Differential Revision: https://reviews.llvm.org/D136249