Rename insertOneElementAt -> insertElementAt.

insertElementsAt() is removed, so "One" in insertOneElementAt() no longer
make much sense. Rename it for brevity.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205372 91177308-0d34-0410-b5e6-96231b3b80d8