[DWARF parser] Cleanup code in DWARFDebugLine.

Move several function definitions into .cpp, unify constructors
and clear() methods (fixing a couple of latent bugs from copy-paste),
turn static function parsePrologue() into Prologue::parse().

More work needed here to untangle weird multiple inheritance
in table parsing and dumping.

No functionality change.

llvm-svn: 207579