[Modules] Move InstIterator out of the Support library, where it had no
business.

This header includes Function and BasicBlock and directly uses the
interfaces of both classes. It has to do with the IR, it even has that
in the name. =] Put it in the library it belongs to.

This is one step toward making LLVM's Support library survive a C++
modules bootstrap.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202814 91177308-0d34-0410-b5e6-96231b3b80d8