[flang] Change parsing of ambiguous array-spec

An array-spec like `(:,:)` (with one or more colons) is either a
deferred-shape-spec-list or an assumed-shape-spec-list and they
can only be distinguished by context that the parser doesn't have.

We were parsing these as assumed-shape-spec-list but they are easier
to deal with if we parse them as deferred-shape-spec-list because
anything that is the latter is also one of the former.

Original-commit: flang-compiler/f18@78c3f3b96f78e45b1b9966b331d8cda691fa39bd
Reviewed-on: https://github.com/flang-compiler/f18/pull/630
Tree-same-pre-rewrite: false