[ObjC] Fix function signature handling for blocks literals with attributes

Block literals can have a type with attributes in its signature, e.g.
ns_returns_retained. The code that inspected the type loc of the block when
declaring its parameters didn't account for this fact, and only looked through
paren type loc. This commit ensures that getAsAdjusted is used instead of
IgnoreParens to find the block's FunctionProtoTypeLoc. This ensures that
block parameters are declared correctly in the block and avoids the
'undeclared identifier' error.

rdar://35416160

llvm-svn: 317736