[Sema][ObjC] Make sure -Wblock-capture-autoreleasing issues a warning

even in the presence of nullability qualifiers.

This commit fixes bugs in r285031 where -Wblock-capture-autoreleasing
wouldn't issue warnings when the function parameters were annotated
with nullability qualifiers. Specifically, look through the sugar and
see if there is an AttributedType of kind attr_objc_ownership to
determine whether __autoreleasing was explicitly specified or implicitly
added by the compiler.

rdar://problem/30193488

llvm-svn: 293194