[llvm-objcopy] Replace the size() helper with SectionTableRef::size

Summary:
BTW, STLExtras.h provides llvm::size() which is similar to std::size()
for random access iterators. However, if we prefer qualified
llvm::size(), the member function .size() will be more convenient.

Reviewers: jhenderson, jakehehrlich, rupprecht, grimar, alexshap, espindola

Reviewed By: grimar

Subscribers: emaste, arichardson, jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60028

llvm-svn: 357347