[flang][openacc][NFC] Make self clause value optional in ACC.td and extract the parser

Set the isOptional flag for the self clause. Move the optional and parenthesis part of the parser. Update the rest of the code to deal with the optional value.

Preparatory work for D106968.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D106965