[DebugInfo/DWARF] [4/4] Unify handling of compile and type units. NFC

This is patch 4 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 4 combines separate DWARFUnitVectors for compile and type units
into a single DWARFUnitVector that contains both.  For now the
implementation distinguishes compile units from type units by putting
all compile units at the front of the vector, reflecting the DWARF v4
distinction between .debug_info and .debug_types sections.  A future
patch will change this to allow the free mixing of unit kinds, as is
specified by DWARF v5.

Differential Revision: https://reviews.llvm.org/D49744

llvm-svn: 338633