[DebugInfo] Make most debug line prologue errors non-fatal to parsing

Many of the debug line prologue errors are not inherently fatal. In most
cases, we can make reasonable assumptions and carry on. This patch does
exactly that. In the case of length problems, the approach of "assume
stated length is correct" is taken which means the offset might need
adjusting.

This is a relanding of b94191fe, fixing an LLD test and the LLDB build.

Reviewed by: dblaikie, labath

Differential Revision: https://reviews.llvm.org/D72158