[Sema] Improve -Wstrict-prototypes diagnostic message for blocks.

Print "this block declaration is not a prototype" for non-prototype
declarations of blocks instead of "this function declaration ...".

rdar://problem/32461723

Differential Revision: https://reviews.llvm.org/D33739

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@304507 91177308-0d34-0410-b5e6-96231b3b80d8