llvm-objdump: speed up -objc-meta-data

Running a Debug build of objdump -objc-meta-data with a large Mach-O file is
currently unnecessarily slow.

With some local test input, this change reduces the run time from 75-85s down
to 15-20s.

The two changes are:
  Assert on pointer equality not array equality
  Replace vector<pair<address, symbol>> with DenseMap<address, symbol>

Additionally, use a std::unique_ptr rather than handling the memory manually.

Patch by Dave Lee!

llvm-svn: 291398