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

Fix some uninstantiable code in ASTVector::insert. I've added a
cheap-and-dirty compile test for this, because I don't have the time to
figure out a nice way to get a real ASTContext to implement executable
tests - but we probably should have them for this ADT.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186253 91177308-0d34-0410-b5e6-96231b3b80d8