[Sema] Put nullability fix-it after the end of the pointer.
Fixes nullability fix-it for id<SomeProtocol>. With this change
nullability specifier is inserted after ">" instead of between
"id" and "<".
Reviewers: jordan_rose, doug.gregor, ahatanak, arphaman
Reviewed By: jordan_rose
Differential Revision: https://reviews.llvm.org/D38327