Add empty() to atom_collection.

"x.empty()" is more idiomatic than "x.size() == 0". This patch is to
add such method and use it in LLD.

Differential Revision: http://llvm-reviews.chandlerc.com/D3279

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205558 91177308-0d34-0410-b5e6-96231b3b80d8