[flang] Allow modification of construct entities

[flang] Allow modification of construct entities

Construct entities from ASSOCIATE, SELECT TYPE, and SELECT RANK
are modifiable if the are associated with modifiable variables
without vector subscripts.  Update WhyNotModifiable() to accept
construct entities that are appropriate.

A need for more general error reporting from one overload of
WhyNotModifiable() caused its result type to change to
std::optional<parser::Message> instead of ::MessageFixedText,
and this change had some consequences that rippled through
call sites.

Some test results that didn't allow for modifiable construct
entities needed to be updated.

Differential Revision: https://reviews.llvm.org/D123722