[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 "<".

rdar://problem/34260995

Reviewers: jordan_rose, doug.gregor, ahatanak, arphaman

Reviewed By: jordan_rose

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D38327

llvm-svn: 314473