[Object, MachO] Introduce MachOObjectFile::load_commands() range iterator.

Summary:
Now users don't have to manually deal with getFirstLoadCommandInfo() /
getNextLoadCommandInfo(), calculate the number of load segments, etc.

No functionality change.

Test Plan: regression test suite

Reviewers: rafael, lhames, loladiro

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10144

llvm-svn: 238983