Rename is_*_v4_or_v6 to *_is_v6.

[r12f]

This makes code more explicit on 0 = v4 and 1 = v6.

This change will not change any generated code. Here is the diff for proof. Only comment update after make:

r12f@r12f-dl380:~/data/code/sonic/DASH/dash-pipeline
$ diff SAI/SAI/experimental/ ~/data/code/sonic/DASH-exp/dash-pipeline/SAI/SAI/experimental/

$ diff SAI/lib ~/data/code/sonic/DASH-exp/dash-pipeline/SAI/lib/
diff SAI/lib/saidashoutboundcatopa.cpp /home/r12f/data/code/sonic/DASH-exp/dash-pipeline/SAI/lib/saidashoutboundcatopa.cpp
47c47
<         // set is_ipv6_field_id field
---
>         // set v4_or_v6 field
241c241
<         // set is_ipv6_field_id field
---
>         // set v4_or_v6 field
diff SAI/lib/saidashoutboundrouting.cpp /home/r12f/data/code/sonic/DASH-exp/dash-pipeline/SAI/lib/saidashoutboundrouting.cpp
45c45
<         // set is_ipv6_field_id field
---
>         // set v4_or_v6 field
136c136
<                     // set is_ipv6_field_id field
---
>                     // set v4_or_v6 field
151c151
<                     // set is_ipv6_field_id field
---
>                     // set v4_or_v6 field
166c166
<                     // set is_ipv6_field_id field
---
>                     // set v4_or_v6 field
181c181
<                     // set is_ipv6_field_id field
---
>                     // set v4_or_v6 field
196c196
<                     // set is_ipv6_field_id field
---
>                     // set v4_or_v6 field
211c211
<                     // set is_ipv6_field_id field
---
>                     // set v4_or_v6 field
226c226
<                     // set is_ipv6_field_id field
---
>                     // set v4_or_v6 field
328c328
<         // set is_ipv6_field_id field
---
>         // set v4_or_v6 field

[r12f]

Thanks a lot for the quick review, Marian and Chris!