[DebugInfo] Use Cursor to detect errors in debug line prologue parser

Previously, the debug line parser would keep attempting to read data
even if it had run out of data to read. This meant errors in parsing
would often end up being reported as something else, such as an unknown
version or malformed directory/filename table. This patch fixes the
issues by using the Cursor API to capture errors.

Reviewed by: labath

Differential Revision: https://reviews.llvm.org/D83043