Use HashSet<std::string> instead of unordered_set<>.

Change the default parameters for HashSet<std::string> to
allow passing StringPiece as a key, avoiding an unnecessary
allocation. Use the HashSet<std::string> instead of
std::unordered_set<std::string>. Rename HashSet<> functions
that mirror std::unordered_multiset<> to lower-case.

Fix CompilerDriver::LoadImageClasses() to avoid using
invalidated iterator.

Test: m test-art-host-gtest
Test: testrunner.py --host
Change-Id: I7f8b82ee0b07befc5a0ee1c420b08a2068ad931e