Return a vector rather than mutating a given one.

This is cleaner and as efficient as before.

Differential Revision: http://llvm-reviews.chandlerc.com/D3284

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205590 91177308-0d34-0410-b5e6-96231b3b80d8