[DebugInfo/DWARF] [3/4] Rename DWARFUnitSection to DWARFUnitVector. NFC

This is patch 3 of 4 NFC refactorings to handle type units and compile
units more consistently and with less concern about the object-file
section that they came from.

Patch 3 simply renames DWARFUnitSection to DWARFUnitVector, as the
object-file section of a unit is nearly irrelevant now.

Differential Revision: https://reviews.llvm.org/D49743

llvm-svn: 338632