[flang][OpenMP] Rework LINEAR clause (#119278)

The OmpLinearClause class was a variant of two classes, one for when the
linear modifier was present, and one for when it was absent. These two
classes did not follow the conventions for parse tree nodes, (i.e.
tuple/wrapper/union formats), which necessitated specialization of the
parse tree visitor.

The new form of OmpLinearClause is the standard tuple with a list of
modifiers and an object list. The specialization of parse tree visitor
for it has been removed.
Parsing and unparsing of the new form bears additional complexity due to
syntactical differences between OpenMP 5.2 and prior versions: in OpenMP
5.2 the argument list is post-modified, while in the prior versions, the
step modifier was a post-modifier while the linear modifier had an
unusual syntax of `modifier(list)`.

With this change the LINEAR clause is no different from any other
clauses in terms of its structure and use of modifiers. Modifier
validation and all other checks work the same as with other clauses.