[DebugInfo] Unify Cursor usage for all debug line opcodes

This is a natural extension of the previous changes to use the Cursor
class independently in the standard and extended opcode paths, and in
turn allows delaying error handling until the entire line has been
printed in verbose mode, removing interleaved output in some cases.

Reviewed by: MaskRay, JDevlieghere

Differential Revision: https://reviews.llvm.org/D81562