[DebugInfo/DWARF] [2/4] Type units no longer in a std::deque. NFC

This is patch 2 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 2 takes the existing std::deque<DWARFUnitSection> for type units
and makes it a simple DWARFUnitSection, simplifying the handling of
type units and making it more consistent with compile units.

Differential Revision: https://reviews.llvm.org/D49742

llvm-svn: 338629