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

This reverts commit b18b043a5a37f76803d89467e46bcac286c0ecae.

Reapply with fix for the configure+make build (missing include of
ASTContext.h).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186257 91177308-0d34-0410-b5e6-96231b3b80d8