[Flang] Add improvements to prefetch directive syntax

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](https://www.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](https://support.hpe.com/hpesc/public/docDisplay?docId=a00115296en_us&page=PREFETCH.html&docLocale=en_US)
>
> https://github.com/llvm/llvm-project/pull/139702#issuecomment-2879950146

Please go through the comments here: https://github.com/llvm/llvm-project/pull/139702

Start by reading the comments on pull request #139702 and the linked Intel and HPE prefetch directive documentation. The issue does not identify files or tests; completion would require reaching agreement on the proposed additional prefetch syntax.

Written by the indexing model from the issue text.