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