[Flang] Add improvements to prefetch directive syntax

[Thirumalai-Shaktivel]

The design discussion starts here:

Just a pass-through comment. IBM and HPE have prefetch directives that have more options. Might be good to check with @kkwli @DanielCChen @tmjbios to see whether they are OK with the syntax in this PR. intel.com/content/www/us/en/docs/fortran-compiler/developer-guide-reference/2024-2/prefetch-and-noprefetch-general-directives.html support.hpe.com/hpesc/public/docDisplay?docId=a00115296en_us&page=PREFETCH.html&docLocale=en_US

#139702 (comment)

Please go through the comments here: #139702

[klausler]

It's not clear what kind of action you want to be taken to close this bug report. Please clarify or close.

[Thirumalai-Shaktivel]

The prefetch directive has a basic implementation which accepts only variable or array item. I'm adding some todo's here

TODO:

• Specify RW, localityType, cacheType in the directive.
• Add support for Array section