Revert "PR16540: ASTVector::insert(Context, Iter, Element) doesn't compile"

This reverts commit r186253.

This is failing to link under Configure+Make on the buildbots for
reasons I don't immediately understand.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186255 91177308-0d34-0410-b5e6-96231b3b80d8