dwarfdump: Add a bit more DWARF64 support

This test case was incorrect because it mixed DWARF32 and DWARF64 for a
single unit (DWARF32 unit referencing a DWARF64 str_offsets section). So
fix enough of the unit parsing for DWARF64 and make the test valid.

(not sure if anyone needs DWARF64 support though - support in
libDebugInfoDWARF has been added piecemeal and LLVM doesn't produce it
at all)

llvm-svn: 361582