[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 "the
claimed length is correct" is taken to be consistent with other
instances such as the SectionParser, which ignores the read length.

Reviewed by: dblaikie

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