widl: Fix detection of non-pointer context handles.

widl: Fix detection of non-pointer context handles.

is_ptr cannot be used because it follows the chain of types into the
type which has the context_handle attribute, which is typically "void *"
and so causes these context handles to be incorrectly detected as
context handles. Instead, we can use is_aliaschain_ptr to follow the
chain of aliases without following pointers and the absence of the
context_handle attribute indicates that it must be present on a type
after following a pointer.