[flang] Allow a generic-spec on a PUBLIC/PRIVATE statement to declare a generic

A generic-spec can appear on a module accessibility control statement
even if it has not been declared as a generic interface, because there's
nothing else that it could be.

While here, simplify the parse tree and parser for AccessId, since
one of its alternatives is ambiguous with the other.

Differential Revision: https://reviews.llvm.org/D134471