When checking for nonnull parameter attributes, also check the ParmVarDecl since the attribute may reside there, instead of just on the FunctionDecl. Fixes PR21668.
AST: Incomplete types might be zero sized

Comparing the address of an object with an incomplete type might return
true with a 'distinct' object if the former has a size of zero.
However, such an object should compare unequal with null.