Remove unused enum Position::ANY and third parameter of insertElementAt().

insertElementAt()'s third parameter is not only unused but also ignored
if you pass Position::END. The actual meaning of the parameter was obscure.

Differential Revision: http://llvm-reviews.chandlerc.com/D3256

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205376 91177308-0d34-0410-b5e6-96231b3b80d8