[DWARF] Factor out a DWARFUnitHeader class. NFC

Extract information related to a "unit header" from DWARFUnit into a
new DWARFUnitHeader class, and add a DWARFUnit member for the header.
This is one step in the direction of allowing type units in the
.debug_info section for DWARF v5.

Differential Revision: https://reviews.llvm.org/D46707

llvm-svn: 332289