[Flang] Add parser support for prefetch directive (#139702)
Implementation details:
* Recognize prefetch directive in the parser as `!dir$ prefetch ...`
* Unparse the prefetch directive
* Add required tests
Details on the prefetch directive:
`!dir$ prefetch designator[, designator]...`, where the designator list
can be a variable or an array reference. This directive is used to
insert a hint to the code generator to prefetch instructions for
memory references.