[flang][directive] Enforce basic semantic check for all clauses

This patch is a follow up to D94821 to ensure the correct behavior of the
general directive structure checker.
This patch add the generation of the Enter function declaration for clauses in
the TableGen backend.
This helps to ensure each clauses declared in the TableGen file has at least
a basic check.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D95108