[analyzer] Don't warn for returning void expressions in void blocks.

This was slightly tricky because BlockDecls don't currently store an
inferred return type. However, we can rely on the fact that blocks with
inferred return types will have return statements that match the inferred
type.

<rdar://problem/13665798>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179699 91177308-0d34-0410-b5e6-96231b3b80d8