Don't allow defining a block with a non-prototype type. Remove a

Don't allow defining a block with a non-prototype type.  Remove a
hack which introduces some strange inconsistencies in compatibility
for block pointers.

Note that unlike an earlier revision proposed on cfe-commits, this patch
still allows declaring block pointers without a prototype.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@73041 91177308-0d34-0410-b5e6-96231b3b80d8