[DebugInfo] Check for errors when reading data for extended opcode

Previously, if an extended opcode was truncated, it would manifest as an
"unexpected line op length error" which wasn't quite accurate. This
change checks for errors any time data is read whilst parsing an
extended opcode, and reports any errors detected.

Reviewed by: MaskRay, labath, aprantl

Differential Revision: https://reviews.llvm.org/D80797