[DWARF] DW_TAG_imported_unit is not a unit type.

As pointed out by David in D38453 and confirmed with the DWARF mailing
list, DW_TAG_imported_unit is not a valid unit type.

llvm-svn: 315244

llvm/include/llvm/BinaryFormat/Dwarf.h

@@ -345,7 +345,6 @@ inline bool isUnitType(dwarf::Tag T) {
   case DW_TAG_type_unit:
   case DW_TAG_partial_unit:
   case DW_TAG_skeleton_unit:
-  case DW_TAG_imported_unit:
     return true;
   default:
     return false;