[CodeComplete] Fix crash when completing ObjC block parameter with a broken type

Summary:
The fix isn't great, but it's hard to fix properly because the completion
code sensibly uses ParmVarDecl to represent parameters, but the AST-building
code sensibly doesn't synthesize them if the type is broken.
Also this case is apparently really rare, so it's probably not worth bending
over backwards for.

Reviewers: ilya-biryukov

Subscribers: javed.absar, kristof.beyls, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60258

llvm-svn: 357686