[flang] Perform more checks on array-specs

There are many constraints on what kind of array-specs can appear
in what contexts. Add `CheckArraySpec()` to perform most of them.
When the check fails, don't set the shape of the symbol being
declared and instead set the Error flag so we can avoid cascading
errors.

Fixes flang-compiler/f18#609.

Original-commit: flang-compiler/f18@f159d97f1f1e02b4d66e410b0f5a587f3c8fc51a
Reviewed-on: https://github.com/flang-compiler/f18/pull/630
Tree-same-pre-rewrite: false