[flang] Fix CONTIGUOUS attribute for construct entities

Currently, the selector of a construct entity (e.g., ASSOCIATE(x=>a(1:20)))
is inheriting the CONTIGUOUS attribute from its associated variable
even if it has subscripts that make it noncontiguous (a(1:20:2)).
Add construct entities to the dynamic contiguity predicate instead.

Differential Revision: https://reviews.llvm.org/D145114