[flang][openacc] Use TableGen to generate the clause parser

This patch introduce an automatic generation of the clause parser from the TableGen
information.

New information can be stored directly in the TableGen file:
- The different aliases that a clause support.
- prefix before a value.
- whether a prefix is optional or not.

Makes it easier to add new clauses and also avoid some error (`write` clause incorrect until now).

This patch is updating only the OpenACC part. A patch with a modification of the OpenMP clause parser will follow.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D106968