Rename getInputGraph() and getNextFile().

Seems getSomething() is more common naming scheme than just a noun
to get something, so renaming these members.

Differential Revision: http://llvm-reviews.chandlerc.com/D3285

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205589 91177308-0d34-0410-b5e6-96231b3b80d8
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
Do not use temporary variables to pass them to forEachUndefines.

So that it's obvious that we pass these callbacks only to forEachUndefines.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205593 91177308-0d34-0410-b5e6-96231b3b80d8