[Clacc] Fix some typos

          documented in `README-OpenACC-status.md` about the limited visibility

          of an `acc routine` directive.

* Elsewhere:

    * *exp* `acc routine` -> `omp declare target` plus an

      `omp end declare target` following the associated function declaration.

    * *exp* `acc routine` -> *exp* `omp declare target` plus an

      *exp* `omp end declare target` following the associated function

      declaration.

    * The translation discards *exp* `gang`|`worker`|`vector`|`seq`.

      Notes:

        * These clauses appear to have no counterpart in OpenMP.

    transformACCClauses(D, OMPD_target_exit_data, TClauses, TClausesEmptyCount);

    // Neither acc exit data nor omp target exit data have associated

    // statements, but for some reason OMPTargetEnterDataDirective expects one.

    // statements, but for some reason OMPTargetExitDataDirective expects one.

    assert(!D->hasAssociatedStmt() &&

           "expected no associated statement on 'acc exit data' directive");

    getSema().ActOnOpenMPRegionStart(OMPD_target_exit_data, nullptr);