[C++11] Introduce ObjectFile::sections().

Summary:
This adds ObjectFile::section_iterator_range, that allows to write
range-based for-loops running over all sections of a given file.
Several files from lib/ are converted to the new interface. Similar fixes
should be applied to a variety of llvm-* tools.

Reviewers: rafael

Reviewed By: rafael

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D3069