[C++11] Introduce ObjectFile::symbols() to use range-based loops.

Reviewers: rafael

Reviewed By: rafael

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D3081

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204031 91177308-0d34-0410-b5e6-96231b3b80d8