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

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@293194 91177308-0d34-0410-b5e6-96231b3b80d8