[hmaptool] Fix string decoding for Python 3

[smeenai]

Summary

Our "strings" were actually bytes, which made verbose dumping fail.
Decode them so they actually become strings.

[bruno]

LGTM

clang/utils/hmaptool/hmaptool

def __init__(self, num_entries, buckets, strtable):
         self.buckets = buckets
         self.strtable = strtable
 
     def get_string(self, idx):
         if idx >= len(self.strtable):
             raise SystemExit("error: %s: invalid string index" % (
                     idx,))
         end_idx = self.strtable.index(0, idx)
-        return self.strtable[idx:end_idx]
+        return self.strtable[idx:end_idx].decode()
 
     @property
     def mappings(self):
         for key_idx,prefix_idx,suffix_idx in self.buckets:
             if key_idx == 0:
                 continue
             yield (self.get_string(key_idx),
                    self.get_string(prefix_idx) + self.get_string(suffix_idx))