[CodeComplete] Provide completion in decls even for incomplete types

Summary:
This change fixes lack of completions in the following case
('^'designates completion points) :

    void f(^);
    struct Incomplete;
    Incomplete g(^);

Reviewers: bkramer, aaron.ballman, sammccall

Reviewed By: aaron.ballman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D46639

llvm-svn: 332244