NFC: An iterator for stepping through CodeView type stream in llvm-readobj

This is a small refactoring step toward moving CodeView type stream logic from llvm-readobj to a library. It abstracts the logic of stepping through the stream into an iterator class and updates llvm-readobj to use that iterator. This has no functional change; llvm-readobj produces identical output.

The next step is to abstract the parsing of the different leaf types and then move that and the iterator into a library.

Since this is my first contrib outside LLDB, please let me know if I'm messing up on any of the LLVM style guidelines, idioms, or patterns.

Differential Revision: http://reviews.llvm.org/D19746

llvm-svn: 268334