[DebugInfo] Fix DebugLine::Prologue::getLength

The function a) returned 32-bits when in DWARF64, the PrologueLength
field is 64-bits in size, and b) didn't work for DWARF version 5.

Also deleted some related dead code. With this deletion, getLength is
itself dead, but another change is about to make use of it.

Reviewed by: probinson

Differential Revision: https://reviews.llvm.org/D73626