Expose which_sha implementation to avoid duplicated in CBMC

The `which_sha` function is static so it was copied to [make_common_data_structures.c](https://github.com/aws/aws-encryption-sdk-c/blob/master/verification/cbmc/sources/make_common_data_structures.c). In #638 this was updated and we discussed the possibility of exposing it to avoid duplication.

Start by locating the static which_sha definition and comparing it with verification/cbmc/sources/make_common_data_structures.c. Review the discussion in #638, then verify that the CBMC sources use the exposed implementation without duplicated logic and that verification still builds.

Written by the indexing model from the issue text.