Fix TryParsePtrOperatorSeq.

The syntax rules for ptr-operator allow attributes after *, &,
&&, therefore we should be able to parse the following:

void fn() {
    void (*[[attr]] x)() = &fn;
    void (&[[attr]] y)() = fn;
    void (&&[[attr]] z)() = fn;
}
However the current logic in TryParsePtrOperatorSeq does not consider
the presence of attributes leading to unexpected parsing errors.

Moreover we should also consider _Atomic a possible qualifier that can
appear after the sequence of attribute specifiers.