Add hash map, reduce excessive hashing

Changed the class def index to use a HashMap instead of unordered_map
so that we can use FindWithHash to reduce how often we need to compute
hashes.

Fixed a bug in ClassLinker::UpdateClass where we didn't properly
handle classes with the same descriptor but different class loaders.
Introduced by previous CL.

Before (fb launch):
1.74% art::ComputeModifiedUtf8Hash(char const*)

After:
0.95% art::ComputeModifiedUtf8Hash(char const*)

Bug: 18054905
Bug: 16828525

Change-Id: Iba2ee37c9837289e0ea187800ba4af322225a994