[flang] Add Symbol::IsArray()

Add and use IsArray for symbols, meaning: it is an object entity
with shape.

Original-commit: flang-compiler/f18@31c1a37f03493c7ab56122986f07bb67153b3d91
Reviewed-on: https://github.com/flang-compiler/f18/pull/277
Tree-same-pre-rewrite: false