[DebugInfo] Make incorrect debug line extended opcode length non-fatal

It is possible to try to keep parsing a debug line program even when the
length of an extended opcode does not match what is expected for that
opcode. This patch changes what was previously a fatal error to be
non-fatal. The parser now continues by assuming the the claimed length
is correct, even if it means moving the offset backwards.

Reviewed by: dblaikie

Differential Revision: https://reviews.llvm.org/D72155