[Wdocumentation] fixes an assertion failure with typedefed function and block pointer

Summary:
The assertion happens when compiling with -Wdocumentation with variable declaration to a typedefed function pointer. I not too familiar with the ObjC syntax but first two tests assert without this patch.

Fixes https://bugs.llvm.org/show_bug.cgi?id=42844

Reviewers: gribozavr

Reviewed By: gribozavr

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66706

llvm-svn: 370677