[dwarfdump] Fix spurious verification errors for DW_AT_location attributes

Verifying any DWARF file that is optimized and contains at least one tag
with a DW_AT_location with a location list offset as a
DW_AT_form_dataXXX results in dwarfdump spuriously claiming that the
location list is invalid.

Differential revision: https://reviews.llvm.org/D40199

llvm-svn: 325430